The Endurance Test

Tyler tapped the floor, signaling that it was now or never for Emily to dismount. She tried to, but since she was small, and splayed awkwardly to ride his wide hips, and there was a lot of him to try to pull herself off of, she didn't make it. His instincts demanded that he grab her hips and pull her to him, no matter how it would hurt her, maybe even damage her. He resisted the urge, barely, but gravity alone was enough. She slipped back down on him, and he swelled up inside her, making her mewl at the strain. Then it erupted into her. They could both could feel the pressure it built up inside of her, with him plugging her tight. She reached climax as well, the contractions redoubling around him, squeezing down on him even harder. She cried out, unable to suppress it, and he couldn't choke back a groan.

Brian saw all this happening. It was obvious to everyone what had happened. It took Brian's concentration off of the million mundane things he had to think about, to keep himself from climaxing.

"Wow," he muttered. "That's hot."

He could feel Beth's ardor as well, but she turned back to him.

"That makes you the last man standing," she said. "Bravo."

It reminded Brian that this had all started with his embarrassment at having ejaculated too quickly with Emily. It gave him a sense of pride that he had outlasted the other guys by now. It would be so easy to let go inside of Beth, as Mika already had. His erection was starting to get sore from staying hard so long. But he wanted to win fair and square. He just barely managed to hold out until the timer went off.

Beth got up and helped Emily dismount, though Tyler was starting to soften, having just released his pent-up load.

Lilly moved over to her brother again. This time, there wasn't a question of whether they would go through with it. They had already crossed this line, and she was far enough gone that she found herself looking forward to taking him inside her again. She tried not to think about the fact that after being excited so long, he was dripping with precum.

"Remember what I said before?" Lilly asked.

"No semen in your vagina," Brian said.

"Final round, 20 minutes," Beth said. "If you can make it 20 minutes, you can make it all night."

Beth knelt across Tyler's lap and counted down before starting the timer. Then she guided him into her. It was easier this time, with Tyler having just spent himself inside of Emily.

"Mm, I see you're recovered already," Emily said to Mika with a smile of delight as she sat down upon his prick.

"Already? That was more than 15 minutes ago," Mika said.

"Well, cumulatively you didn't even make it 18 minutes before," she teased.

"Are you saying you want me to fail again?" He smirked.

"You know I wouldn't mind if you did," Emily replied. "On the other hand, maybe you could also use some practice after all."

The other couples were comfortably conjoined with each other. Not so with Brian and Lilly. They were conjoined, but not comfortable with it.

Lilly tried to cover her gasp of breath, saying, "Ugh. We definitely shouldn't be doing this."

"Then get up off me if you want to," Brian said, though he didn't mean it.

"Fuck you."

"I bet you would."

"This isn't fucking," she said. "And maybe you should stop talking dirty, before you get yourself too excited."

If talking was to be their undoing, then not talking was even worse. As the distracting conversation lulled, she could feel every throb, every shift of Brian's cock within her. She was so sensitive by now, that every sensation gave her shivers. He was positioned just right to press against something tender deep inside. It would be so easy to set her off.

"Stop doing that," she said.

"Doing what?"

"You know exactly what I'm talking about," she whispered, not that it would do any good. Just then his cock twitched inside her, making her pussy spasm. "That."

"Me? You're the one trying to get me off inside you, even though you said not to," he said.

"I am not," she whined.

Brian looked up his sister again, naked, on top of him, his cock in her hot, wet vagina. He had never really thought of her like this. It had always been the furthest from his mind. Heck, anyone who remotely reminded him of Lilly had always been a turn-off. But now he felt foolish for that.

Her large, plump breasts hung over him. He longed to caress them. Without thinking, he reached one hand up toward her. A sigh escaped her lips when his fingers brushed her nipple, and he felt it down to her core. Then she remembered herself.

"What are you doing?" she asked.

"Just tickling you," he said, thinking that was at least less sexual. He tickled her belly, and it made her abdominal muscles contract.

"Not a good idea," she said.

To the others, twenty minutes was starting to feel a bit long, maybe a bit boring. To the siblings, twenty minutes was feeling long because they weren't going to last that long.

Lilly could feel the tension building up in her. She knew she couldn't hold it back forever, and this was starting to feel like forever.

"I...I'm gonna cum. Any second now," she warned Brian, her breathing getting heavier, her voice rising. "Can you handle it?"

"I dunno," he said.

"Not good enough. I need a real answer, and I need it like right now!" she said. "If you have to tap out, you gotta do it now, or I'm gonna lose it."

"I think I can..." he said. He wasn't entirely sure, but he didn't want this to end so soon if it didn't have to.

"Here...here it...comes! Oh fuck!"

Her vaginal walls were starting to squeeze in time, and then her entire body tensed up around him, holding him tight to her, tight inside her. She cried out and fell forward onto her elbows, her breasts pressed hotly against his chest, her panting whimpers in his ear. She knew that the orgasm she was experiencing was already more intense than any she'd had with Mika or Tyler, and it made her ashamed to show it.

Everything she was reacting to, he could feel happening inside her, with perfect clarity. Her pussy was wringing and caressing him inside her.

"I'm sorry...I can't..." he said. He tapped the floor to signal that he was about to lose control, but Lilly was in no state to pull herself off him.

She felt him throb once more, swelling larger and longer and fuller, and then he twitched inside her, and his semen jetted forcefully into her, hitting her cervix. Her hips bucked, instinctively realigning herself with the influx of seed. Her vaginal contractions redoubled, and her cries got louder and higher, at least when she had the breath for them.

When his sister's pussy started milking him of his seed, it pushed Brian's orgasm to greater heights. He swelled up inside her again, delivering another jet of it, feeling the warmth of it spread out and disappear, drawn deep within her, causing another wave of aftershocks in her, making her clench down on him again, making him pulse and well again. He had never experienced anything like this before. He found himself ejaculating more than he thought he possibly could, as their shared climax went on and on, feeding back into itself.

Eventually Brian had no more to give, and Lilly's muscles turned to jelly inside and out, though she was still quivering and sensitive. She knew she ought to feel sullied, having her own brother's semen inside of her. The one thing she had hoped to avoid. Maybe the regret would come later, but in that moment, she was too awash in pleasure and tactile sensations and hormones and pheromones.

There was a level to her satisfaction that she couldn't put a name to, because she didn't know yet what it signified. But it went beyond feeling relieved of her tension and need, like sex had always made her feel. It went beyond feeling warm and full, from having a man ejaculate inside her. She had unknowingly given in to a primal drive she didn't realize she had.

When her senses returned, she looked to the side, where the others were watching them.

"Holy shit, you two," Emily said. "That was fucking hot."

It made Lilly laugh, though her laugh was a little like choking back a sob. But Brian laughed too.

Then Beth's timer went off. There was an awkward moment as none of the girls wanted to get up off their partners. But the endurance test was done.

As usual, Beth led the group. She started to move her hips forward and back, in long strokes. Tyler grabbed her thighs, and then she leaned over him, kissing as they pushed against each other.

Likewise, Emily started bouncing her short, slim body on Mika's cock, seeking out her pleasure again. Mika held her hands in his.

That left Brian and Lilly.

"How are you still hard?" she asked.

"I dunno," he said. "It sure felt like I was done a second ago, but..."

She started to roll back and forth, feeling him rub against her vaginal walls, swimming in their combined fluids.

"Listen. This isn't fucking. We are not fucking," she said. She didn't know what else to call it, she just wasn't ready to admit to herself how far they had already gone.

"Of course not," he said, finally grabbing her hips like he'd wanted to all night, and pressing himself eagerly up into her. He could feel her body reacting to his. It made him want more of that.

Next to them, Emily started to whimper again as she came, whimpering to Mika to cum inside her, which he gladly did.

Tyler rolled Beth over onto her back, and plowed into her with his full length. He slammed his body into hers until neither of them could take it any longer. Pushing himself into her as hard as he could, his veins bulged with the strain as he filled her again, making her cum again as she clung to him with her arms and legs.

Lilly and Brian, having just climaxed, took their time with it, relishing every little movement, every little sensation of their bodies against each other. They were still there, rutting on the carpet, driven by a shared illicit purpose, while the others caught their breath and disentangled their sweaty, exhausted limbs.

Emily got up, leaning on the furniture for support while her legs wobbled under her. She was well-used, with two guys' loads in her pussy, and having taken Tyler for the first and hopefully not last time.

"Get a room, you two," she joked, as she went to wash up and left them to it. After all, for all she knew, Lilly and Brian had been building up to this for months. Maybe even their entire lives.

"I'm about to cum again," Brian eventually warned his sister. "Do you...uh...want your brother's semen in your vagina?"

"Do I really have to answer that?" Lilly gasped.

"Kinda, yeah," Brian said, his desperation growing.

"Okay...fuck...yeah...I want it," she said, struggling to say it.

"Want what?" he asked with a smirk.

"Asshole."

"I didn't know...that was...an option," he joked. "Oh god!"

He wrapped his arms around her, holding her tight to him as he lost control. It was different this time, moving with intention, ejaculating inside his sister's pussy on purpose. Feeling it put her over the edge, and she pressed down on him, pushing him deeper up into her. They stayed affixed together, as their bodies exchanged vital fluids again. Lilly's eyes rolled back in her head as she rode out the bliss of that illicit union. Brian gave more than he thought possible, and then some, the waves of his pleasure cresting and reflecting off of her. He didn't know sex could be like this, let alone with the last person in the world he should be doing it with.

Afterwards, they were immobile for a while as they recovered. Somewhere along the way, everyone else had cleared out and gone to bed--maybe to their own or maybe to each other's, leaving Lilly and Brian alone in the living room.

Suddenly the fact that they'd be sharing a bedroom tonight struck Lilly as suggestive. It would be so easy to give into this, and let it run its course, whatever the family-breaking consequences would be.

"Don't assume this is ever going to happen again," Lilly said, using her big sister voice again. "And don't you dare tell anyone."

Brian nodded. He got up, feeling sore from spending so much time on the hard floor, even with carpet. He felt sweaty and used and certainly smeared with vaginal fluids--Lilly's as well as Beth's and Emily's--but he didn't have a pool of semen on his lap. It had pretty much all stayed inside her. It wasn't even leaking out now as she stood and moved around. He realized he was staring at his sister's pussy, and looked away, embarrassed. Now they were going to bed, in the room they shared, in their separate beds. But first he needed a shower.

---

After that week, they were on winter break. Pretty much all of them went home for the holidays, except for Beth, who wasn't on good terms with her parents anymore.

Brian almost didn't want to go home for the holidays, because it meant returning to a reality where he and Lilly were just brother and sister again. They spent a week in their parent's house, and neither of them dared cross a line while they were there. The only way Brian got through it was with the knowledge that soon enough they'd be back in their high-rise apartment with friends who accepted and even encouraged Brian and Lilly hooking up.

When they returned after winter break, that night hung over them all like a shared fever dream, but mostly things went back to normal in the household. Lilly had her nights with Tyler, and occasionally Mika. Brian had his nights with Emily, and sometimes Beth. They all went back to using condoms again.

Not everything went back to normal though. Regardless what she'd said, Lilly would use her brother for a quickie sometimes. Or sometimes they'd lay around, genitally conjoined, and see who would be the first to give in to the urge to move. Usually it was Lilly.

It wasn't as though she was anything like celibate before, but something had changed now, and not just her relationship with her brother. She still went to Mika or Tyler some of the time--each of the guys was unique, and met a particular need in her--but the same was now true of Brian too. Sometimes what she needed was something only he could provide.

Brian felt much the same way. He still had a huge crush on Emily. He still felt like he was beyond lucky whenever Beth chose him for the night. But sex with Lilly was intense beyond anything he had ever imagined.

Maybe it was the taboo. Every moment they were together, he was still questioning whether they should be doing it, or whether they should stop, even if he never could. It wasn't something he could take for granted. She told him never to expect it to happen again, and he tried never to expect it, so when it did, it was always an unexpected surprise.

But his attraction to her was more than that too. It was also something special in the way she was built. The way they fit together. The woman she had grown up into.

One Saturday, Brian awoke to the feeling of Lilly pulling his boxers down off his hips, exposing his morning wood.

"Uh...good morning," he said groggily, rolling over towards her and lifting his hips to help her finish the job. He had nowhere to be in the morning, and felt amenable to being used. The thought alone brought him to full hardness.

"Rise and shine," she said, with little cheer, as she climbed on the bed with him, putting a knee to either side of his hips. She started stroking the head of his cock up and down her labia. He could feel that she was already hot and wet with arousal, and of course he longed to feel it all around him, but he also knew the rules.

"Shouldn't we be using a condom?" he asked. Not that he worried what would happen if they didn't. But while the others were just fine with the siblings fucking, fucking bareback they would still take issue with.

"Let's just say you've already got me in a state where that won't do any good," she said.

He could feel her getting wetter, feel the heat of her enticing him. It was definitely getting harder to think straight, and as much as he'd like any excuse to enter her bare again, this didn't sound right.

"What do you mean?"

"I missed my period, so I took a test, and...it gave me a plus sign," she said.

Brian was struggling to comprehend. Not just what she seemed to be saying, but also because he didn't expect her to take it like this. "But you...you're on birth control."

"Yeah, and I haven't missed a pill, but it does fail sometimes." She sank down on him, making them both gasp at the pleasure of it.

"Holy shit," Brian muttered. "But you said...why me? It could have been anybody's."

Brian still remembered his sister taking Tyler's long cock, slippery with precum, all the way up into her. He remembered her sitting on Mika's semen-covered lap.

"It's possible, but you were the one who ejaculated inside me twice that night."

The words made Brian's cock twitch inside her, viscerally remembering that too.

"I hate to say it, but house rules probably still apply," Brian said.

"We'll see if it matters. I don't think they're going to let us stay once we have an infant with us."

"Wait. You're not going to abort?"

"I haven't decided yet," Lilly said.

Before all this, she'd been taking birth control for a reason. No babies now, maybe not ever. She didn't see herself going down that path.

But then she'd gotten pregnant. The night it had happened, she had no idea it was a possibility, or she certainly wouldn't have even taken Brian's cock inside of her. Certainly wouldn't have let him ejaculate inside her twice in a row. The thought that she might have gotten knocked up by her brother hadn't even occurred to her until the next day, and even then she had no explanation for why the idea had even crossed her mind. But then over the course of the next month she started to have her suspicions. The increased sex drive. The tight, sensitive fit of her bras. The heightened appetite that made her put on more weight whether she wanted to or not. Missing her period might not have always been a warning sign; this time it was the last straw.

And as she started to suspect she might be pregnant, she found her attitude towards the idea changing. She used to think that bearing a child was something she might come around to once she was married and stable and like forty years old. But now it was here, now, and she was starting to think, why not?

On one hand, maybe she'd terminate this pregnancy, which was most likely her brother's doing. On the other hand, if not even the pill could prevent it, it'd be a matter of time until it happened again. Condoms weren't perfect either. Maybe she was just destined to be a mother before she graduated college. In that case, why go to a bunch of hassle to delay the inevitable, maybe by a matter of months.

"Shit. Don't I get a choice?" Brian asked.

"No, that's not how it works. I'm going to be the one gestating it inside my body," Lilly said. "You had a choice, and your choice was to irresponsibly ejaculate into your sister's vagina." The way she was rolling her hips softened the harshness of her words.

"I don't think that's an entirely fair assessment..." Brian said.

"That sperm cell was probably living in your body for years before it got into mine. You had plenty of opportunity to stop it then."

Brian felt panicked adrenaline pounding in his head at the thought of Lilly having his baby. Of having living, breathing evidence of their incest on display for everyone to see. Of having a baby to take care of. And yet her words were also getting him incredibly riled up.

Soon he was far past the point of being able to think through it, as their bodies moved against each other, eagerly, urgently, in shared purpose. She was driven by a desire that wouldn't be denied, and Brian bit his lip, desperately holding out as she mashed her clit against him. The bed creaked beneath them, loudly keeping time. It'd be no secret to their roommates what the two of them were up to. Brian could already imagine their knowing smirks; though the others wouldn't know the half of it just yet.