Back Seat Sister Slip

Summary: Brother secretly fucks twin sister in a car full of guys.

Note: This is a Halloween 2021 Contest Story.

Note 2: Thanks to Tex Beethoven for editing this story.

Back Seat Sister Slip

"Are you really going out in public dressed like that?" I asked, gawking at my eighteen-year-old sister Shayla, my twin, dressed in the sluttiest Batgirl outfit I'd ever seen. Her ensemble, if you could call it that, included a sexy pair of black thigh highs that only enhanced her sex appeal, making my cock harden within my far more vanilla Luke Skywalker costume. I mean seriously, her skirt was so short, there was a good inch plus of bare skin showing between the skirt and her lace top stockings!

Fuck, she looked amazing.

Fuck, I hated that the hottest, sexiest girl I knew was flesh and blood and standing right in front of me on the one hand, but untouchable on the other, because she was my sister.

"Of course I am! But are you going out dressed like that?" She shot back sarcastically. She was a popular cheerleader dating a college basketball player, while I was somewhat of a nerd, who was so different from her that almost no one believed we were even siblings... but nevertheless, we were; twins, even.

Before I could come up with a witty response, she glanced down and checked out my fleshy lightsaber tenting out my costume... neither my boxers nor the fragile Skywalker fabric adequately restraining my not so secret salute, which was aimed directly at her... these cheaply made one-piece outfits not good at concealing anything, To make matters worse, it had snaps to open and close the crotch, that also opened and closed down its legs, which were only about four inches long. Didn't the designer know that guys above the age of two didn't ever wear things like this? But Shayla didn't care about that; she only scoffed, "Are you hard?"

"I'm eighteen; I'm always hard," I shrugged, trying to dismiss her obvious implication that my twin sister's hot outfit, her big tits, her nylon-clad legs, and her pink painted toenails were the cause of it. Truth was, I jerked off to her a lot... and to her almost as hot friends... I even used her soiled panties to jerk off with quite often, or with her pantyhose, after a game.

"Brent, I know you jerk off with my panties," she mentioned bluntly, making my eyes go wide.

I'd be a great con man... not... so I just stammered, "I-I-I don't know what you're talking about."

"Y-y-you don't know what I'm talking about?" she mocked... which she often did.

Attempting to regain some of my tattered pride, I responded with zero originality, "No, I don't know what you're talking about."

"Just so you know, the panties you snitched the other day had a big load of Mike's cum soaked into them," she said with a wicked smirk on her face.

My cheeks went an even darker shade of red... any slim chance she didn't know the truth vanished, as I went speechless. I often sniffed... and then sucked... the wetness out of her panties while reading incest erotica, watching faux incest porn and stroking my cock.

Which now made me wonder... had I sucked another guy's cum out of her panties? And then swallowed it? Gross!

"So, my dearest bro, do you usually just sniff the pussy juice out of my soiled panties, or do you actually suck it out of them?" she asked, sidling up to me...my cock aching even though she was shaming me... as I kept my head down to slyly admire her nylon-clad feet... which also turned me on.

"I've also figured out you have a nylon foot fetish," she said, now standing before me... knowingly wiggling her perfectly manicured and painted toenails.

"So?" I shrugged, that latter point not nearly as disturbing as the thought that I must have sucked her boyfriend's cum out of her panties.

"So... and I'm just asking, mind you... nothing at all personal... don't you think jerking off while picturing your hot sister...while using her cum-filled panties to do it... while also visualising her oh-so shapely nylon-encased legs and feet... is... perhaps... just a little fucked up?" she asked with saccharine sweetness.

"You're being ridiculous," I said, sticking with my complete verbal denial even though my expression gave me away.

"And your very own sister isn't the reason you're rock hard...right now?" she asked, astonishing me by unsnapping my crotch and grabbing my hard dick through my boxers!

"Oooooh," I groaned, shocked to feel my sister's hand on my dick.

"Oh wow, my baby brother has a big dick," she taunted. But she was rather astonished, judging from her tone of voice and the expression on her face. She always called me her 'baby brother' because she'd been born three minutes and six seconds before I was, and fuck, I always hated to hear about it.

Trying to regain a few shreds of dignity, even while, surprisingly, her hand remained on my dick, a fantasy I'd savoured hundreds of times... and!... fully aware my eight-inch cock was bigger than most guys'... I taunted back, "Yeah, it's probably bigger than your college boyfriend's."

She was quiet for a few endless seconds before she gave my cock a firm squeeze, and admitted in a soft voice more to herself than to me, "Actually, it's quite a lot bigger."

Stunned by her frank response, I remained speechless as she pulled her panties down, a red thong actually, and shoved them into my shocked, jaw-hanging-open mouth. "Here's a fresh pair for you to use. Or it's not technically fresh, since I came in them while Amanda finger banged me in the car before dropping me off today. And I know you want to fuck her, too."

As I stood there speechless, her indeed wet panties indeed in my mouth, she turned around and strutted away.

Holy fuck... what just happened?!

Amanda was a fiery redhead with green eyes, who always made my cock harden. She was from Ireland, and she had an accent that couldn't help making a cock harden. She also wore expensive and fashionable apparel since her parents were loaded, and her mom was reportedly a runway model. So yes, I most certainly did want to fuck her.

So did my sister and Amanda lez out together? That I'd imagined many times... very vividly... but until now, my sister hadn't ever made any hints she was bi, and she always had a boyfriend older than she was.

My cock raging... my head spinning... and still trying to process the stunning reality of what had just transpired... I pulled the wet thong out of my mouth and sniffed it.

"I knew you'd do that," she said triumphantly, suddenly returning to the living room.

"Hey, I've just learned my sister is a lesbian," I protested, as if that were a perfectly good reason to be sniffing her thong.

She sauntered in front of me, boots now in hand, they were sexy as fuck over-the-knee boots I might add, and explained, "First of all, I'm not a lesbian, I'm bi. I'm fond of both cock and cunt."

Hearing my sister use the 'c' word was almost as wild as her squeezing my cock earlier. Although she was super hot, and she knew how to use her body to get whatever she wanted, she was a feminist, or she at least claimed to be a feminist, having even scolded me a while ago for calling Mrs. Jansen cunt for giving me a B in Chemistry.

"Second, I have a few girls at school who willingly go down on me whenever I text them," she continued, again squeezing my cock with her free hand. She smiled, "And two of them are friends of yours."

"What? Who?" I asked, with a moan, while she slowly rubbed my cock... seemingly very impressed by it.

"Sorry, I don't get girls to eat my pussy in the science lab, or a bathroom, or the cheerleader's dressing room, or a teacher while I'm on top of her desk... and tell," she responded wickedly, making me moan again... and I was now worried I was about to come!

"A teacher? On her desk? And in her classroom?" I asked, that latter visual the most intriguing one.

"Two of them actually," Shayla drawled, using her fingers to roll around the top of my cock head.

"No way," I denied, while I tried to figure out which two teachers they could be. My sister was a lot of things... both good and bad... but a liar wasn't one of them. If she bragged she had girls eating her pussy at school, including friends of mine, and even including teachers... it had to be true. Her fingers on my dick were driving me wild, and I was getting very worried I might come in my boxers!

"Way! Including..." she embellished as she really worked my cock over... was she trying to make me come? "...your favourite teacher."

"Mrs. Walker?" I gasped with a moan. My English teacher, who wore nylons every day, and who seemed to know I had a nylon fetish, because she sat on her desk, legs crossed, often dangling a heel and sometimes even dropping it to the floor, and then often leaving her sexy nylon-clad foot out in the open for a few minutes for me to drool over. (Have I mentioned I always sat in the front row for a better view? No? Then now you know.)

"Maybe, maybe not," she sing-songed. "But I will tell you that whoever it is, she often texts me during her prep period, begging me to leave my class so she can eat me out in her classroom while I'm on on top of her desk."

"Oh God," I moaned, about to come.

"And your friends, well one of them anyway," she continued, now really working my cock over, "has even eaten my pussy inside this very house, while you were in another room doing whatever you were doing."

"Whaaaaat?" I grunted as I did come in my underwear from realizing she could only be talking about Emily, since she was the only friend of ours of the female persuasion who'd ever been in our house.

"You're a very sick boy," she smiled as she kept rubbing my cock through my orgasm, "coming from your big sister stroking your big cock while you imagine our mutual friend 'E' eating her out."

"Fuck," I gasped weakly, while soaking my underwear.

"So, so very sick," she purred, giving my still hard cock a final squeeze, before turning away.

As I stood there in complete shame and awe... totally confused about what this meant... the bottom of my costume still hanging open, since my sister had just given me a hand job... I heard the garage door opening, meaning Mom was home, our dad having left us when we were seven.

Since my sister's thong was still in my hand, I reached under my costume and stashed it inside my underwear... on the side, where it wouldn't get soiled by my cum.

"Creative hiding place," Shayla smiled, as she sat down (with the entirety of her sexy thigh high stockings within view, I'm just saying), then sliding a sexy foot into her equally sexy boot... gazing at me steadily the entire time.

"Yeah... I... Jesus!" I said nonsensically, totally unable to process what had just happened in any way, or to speak in any rational way.

"You've received the gift of my underwear, obviously happily, so now you'll kindly give me your underwear," she ordered, one boot on, and a hand extended.

"What?" I asked, having heard her, but befuddled by her request.

"Hurry up, Mom will be walking in here at any moment," she demanded, waving her hand around in an urgent, bossy fashion.

"Okay, okay," I agreed, urgently pulling them down and off, her thong falling to the floor. I grabbed the thong and handed her my cum-stained underwear, before hurrying to snap myself up, even though I had no idea why I was complying with such an order.

Shayla examined the big load inside them and gasped, "Shit, you shoot buckets!"

Before I could respond, even if I had any idea what to say, we heard the door into the kitchen open.

"I'm home," Mom sang out like she did every day when she got home... which always made me feel warm and safe.

Shayla's thong was still in my hand, and she whispered, this time because she was concerned, and not just because she wanted to control me, "Quick! Hide my thong!"

"Where?" I asked, flying into a sudden panic about getting caught with my sister's soiled panties in my hand, which I needed to do something about, within the few moments before Mom came into the living room!

Shayla stuffed my boxers under a couch cushion while she suggested, "Wrap it around your dick."

"Yeah, right," I said, stretching out the short legs of my costume to do just that, thinking that made perfect sense, since I didn't have any pockets, or anywhere else to stash it.

A second later Mom came into the room, kissed my cheek, and said, after briefly examining my costume, "You look cute, honey."

"Great, that's what we eighteen-year-old guys always go for, Mom, women calling us cute," I said, trying to deflect my anxiety and guilt, but also hating that word. I was actually decent looking, had had a few girlfriends, a few blow jobs, and had even fucked a couple of girls. So I wasn't some virgin nerd... although I still was a nerd, if my academic achievements counted, plus a geek, because of my obsession with science fiction.

"Sorry, you look handsome, then," she corrected herself.

"Mom, really?" I sighed wearily.

"He's fishing for sexy," Shayla pointed out, both of her tall boots now in place on her feet and legs.

Mom turned around to greet her daughter, but said instead, her tone leaping from sweet and motherly to firm and motherly, "You, young lady, are not going out dressed like that!"

"But it's Halloween, Mom," she said, standing up, likely going for intimidating. "By law I have to be dressed like this!"

"Shayla Marie Sorensen," Mom barked, and employing our full names was a quick, clear clue that she wasn't happy with us.

"Mahawam, I'm just going to a party," Shayla said, always annoyed at her mother's response to her fashion sense.

"Not wearing that outfit, you aren't," Mom repeated.

Not wishing to witness the Battle of the Stubborn Females, and knowing this could easily become a lengthy embroilment, I headed off to my room, just as my cell rang.

"Hey," I answered.

"We're here," Steve said.

"Oh, okay," I said, looking at my watch to see it was already nine. Time certainly flies when you're having a surreal kinky encounter with your sister! I had planned on putting on some fresh underwear, but now I didn't have time... while my sister's panties were tangled around my hard cock, that was definitely tenting out my outfit even more without the underwear to help keep it under wraps.

"What about him?" Shayla asked, great at deflecting blame by pointing a finger, in this case literally, and at my dick.

"What about him?" Mom asked, looking back at me, still wearing her nurse's unform... since she worked at one of the few hospitals that still required their nurses to wear all white, including most significantly for me, white pantyhose.

"He's not even wearing any underwear," Shayla said, pointing out my cock, which was unashamedly (unlike the rest of me) pointing right back at the two of them.

Mom looked down at my crotch, and her face went a shade of embarrassed red.

"I-I-I've got to go," I stammered, "Steve is already here."

"Oh good, then he can give me a ride too," Shayla blurted out hurriedly.

Mom, rattled by seeing my erection it seemed, and clearly too tired to endure a marathon battle with her daughter, sighed and caved, "Just go, then. But at least behave yourself!"

"I always do," Shayla said. "Let's go, big brother," she said, bouncing past me... awarding me that honorific for the first time ever, and for a very nefarious reason... and as I followed her out, I wondered if Mom had twigged onto her daughter's penile innuendo.

Once we got outside, I said, "I can't believe you said that."

"It worked," she shrugged. "And it was true."

"You're not wearing any underwear either," I pointed out, as we headed down the walk to Steve's VW beetle.

"I'm not, only because you're wearing them," she said, which I suppose was technically true.

"I'm not wearing...."

Shayla punched me in the shoulder, quipping, "Punch buggy, no return!"

As we reached the car, Shayla popped around to the driver's side and asked, "Hey sexy, can you give me a ride to the Parkers' party?"

Shayla always flirted with all of my friends, especially with Steve, and thus she always got whatever she wanted from them.

"I'd love to, but I only have room for one more passenger," he said sorrowfully, since our other two closest friends, Eddie and Baker, were also in the car.

"No worries, I can just sit on Brent's lap," she said, which made my eyes go wide. This prospect would have been a turn-on even before what had just happened... but now it carried an entire new level of sexy implications!

"Well then, climb on in," Steve said, as he got out of the car to allow first me, and then my sister to clamber into the back seat.

"The Parkers live fifteen minutes out of town," Eddie pointed out, clearly annoyed by this unexpected complication. He was our Sheldon, meaning he always needed to be on time for everything. He also thrived on structure, or rather he obsessed whenever it was threatened. Shayla had likely just placed both of those life requirements at risk.

Once I'd settled into my seat, I quickly adjusted my hard cock, just before my sister sat down in my lap... and directly onto my hard dick... making me groan.

"Are you okay, big brother?" my sister asked solicitously, leaning back against me, which enabled her to grind across my cock... clearly she was enjoying playing the tease.

"Yeah, I'm good," I said, more worried I might come in my costume than I was about any minor discomfort.

"Drive, chauffeur Steve," Shayla ordered imperiously, leaning forward to rub the top of Steve's buzz-cut head, again her body shifting across my cock... which hadn't softened at all after dropping that load, and it likely had no intention of returning back to slumber mode for a while longer. She had to know what she was doing to me!

"Yes, my fair lady," Steve said as suavely as he could manage from his lofty teenaged perch in life, preening under all the attention from my sister he could get. He, or actually all of my friends, had a serious crush on my sister.

"Ooh, aren't you the Prince Charming," she crooned as she leaned back against me... once again her ass slow dancing across my cock... and this time she let out the quietest of moans.

"I'll always be princely for you, milady," Steve said, flirting pathetically over his shoulder as he began driving.

"This detour will make us very late," Eddie sighed.

"That's okay," Baker said.

"It isn't okay! Our entire plan has just been tossed out the window," Eddie disagreed.

"So what are you studs planning for tonight?" Shayla asked, my cock resting directly beneath her pussy.

"We're going to a different party," Baker said, sitting to our right and definitely checking out my sister... a box of beer resting between us.

"Which one?" she asked, swiveling her body around towards Baker, and thus teasing my cock some more.

"Howie's," Baker said. "The plan is to play some dungeons and dragons, and then head out to the midnight showing of the epic The Rocky Horror Picture Show, just like we've done for the past four Halloweens. It's always a blast!"

"No, I won't allow that," she said, wiggling her ass a little... seemingly for my benefit... but as if she was oblivious to what she was doing to my hard cock. "Change of plan: you guys are now attending the party at the Parkers'."