I don't know what possessed me that night, but I found myself standing outside my sister's bedroom door at three in the morning. I hadn't been able to sleep all night, even though I had masturbated twice already. I couldn't stop thinking about her.
I guess it was because of earlier that day, at the pool at our aunt and uncle's house. I had never seen her in a bikini before, and she was wearing this tight little black string number, with small triangles covering her nipples and surprisingly very little covering her pussy. There was so little to it that it was ridiculous. Even though we were both eighteen, she would never have gotten away with it at home, but mom and dad weren't here, and our aunt and uncle let us do whatever we want. I think one time my uncle tried to have a manly talk with me about sex, and something along the lines of how masturbation was normal. I didn't understand a lot of it, but regardless my aunt and uncle are very allowing.
But her bikini...I had never been so horny in my young life. I was so hard I had to stay in the pool, and although I had tried to avoid touching my sister, I think my dick took over a little and I found myself brushing against her. She immediately knew I was hard, but didn't make any movement that would reveal me to our aunt and uncle who were sitting on the patio not far away. She didn't even turn her head, but her hand went right to my crotch. Her palm and fingers feeling my erect penis took me by surprise.
"A little excited, huh?" she said quietly.
I couldn't answer. She kept feeling me up, and I felt like I would shoot if she continued.
"You're...bikini," I managed.
"I'll give you a closer look sometime," she said, and swam off.
I had to wait until everyone went inside, and then I snuck to bathroom and jerked myself to a quick orgasm. When I went to the kitchen for dinner, everyone was still in their swimwear, including my sister. I had to sit at the table quickly as I soon had another throbbing erection. During dinner, as my aunt and uncle were talking, she ran her foot up my leg and to my dick. She gave me a footjob while we ate, and I couldn't help but notice her nipples prominently poking through her bikini top. She had a great set of tits, she could be a model very easily. I had jerked off to the thought of fucking her before, but I figured that was normal. But the thought of her pleasing me sexually was very exciting.
After dinner, I went to the bathroom where I proceeded to jerk off again. When I was close to cumming, the door suddenly opened and my sister entered the bathroom. She closed the door and without a word approached me. She bent down and touched my dick, which jumped at her touch. She lightly ran her fingers up and down my shaft, and circled the head. Just as I couldn't take it any more, her hand wrapped firmly around my dick.
I groaned and let out a thick spurt of semen. It hit her square in the face, but she seemed almost unaffected. The second spurt landed on her chest, and I continued dribbling out cum until my dick was gooey and sticky in her hand. I stared dumbfounded as my sister began rubbing my sperm into her breasts. Then she removed her bikini bottoms and began frigging her pussy right in front of me.
She finally made eye contact with me, and smiled devilishly as her hand increased in speed. Her face began to contort, and she tensed suddenly and arched her back. She tilted her head back while her orgasm overtook her, and halfway through she forced herself to look at me, and it seemed to increase the intensity of her climax.
When it was over, she turned the shower on and got in. Still in shock, I stepped out of the bathroom.
To my shock, my aunt was standing right there. I froze, but my aunt was not angry. She grinned and glanced down at my semi-hard dick. She winked at me and left. I didn't know what to make of that. Like I said, she let us do whatever we wanted, and I guess that included sexual activities.
That night I couldn't sleep. I tried wanking, but it didn't compare to the feel of my sister's touch. I had to feel her skin touch me. I needed to see her again.
So here I was, standing in front of her door with an erection. I dropped my boxers right there and opened the door. It was dark, but I could see the outline of my sister sleeping. I approached quietly, not wanting to wake her. I was not in control of myself, I had somehow become addicted to her sexual touch. I gently got onto the bed and straddled her. She stirred and awoke, at first not sure who was sitting on her chest. When she saw it was me, she did nothing. Was that an invitation? I didn't ask, instead I removed her shirt. Her breasts looked especially sexy in the moonlight. She did not move to stop me. Her eyes stayed locked on mine. I pressed my dick between her breasts, and thrust firmly once. She moaned very quietly.
I was suddenly invigorated, and I forced my dick into her mouth. She yelped a bit, surprised by my sudden force. I thrust myself into her mouth, savoring the feel of her wet warmth surrounding my dick. I began fucking her face, with a vigor that surprised her and me both. She still did not move to stop me, she allowed her head to be manipulated by me. I slammed my hips forward, and pulled her head towards me at the same time, and began a rhythm. I was surprised at how much of my dick I could fit in her mouth, and her gag reflex when I reached her throat provided an exciting arousal. Even her nipples brushing against my buttocks turned me on. I groaned very loudly, and I'm sure our aunt and uncle could have heard it.
I cried out as the first spurt of cum shot down her throat. She choked immediately on it, and I felt her cough it up just as the next spurt came. I felt like I released my entire being into her mouth. It had been almost animalistic, and when I was finished, I crouched over her face in exhaustion. I felt her hand lightly stroke my buttocks, while my dick was still lodged in her throat. Her gagging had stopped, once she was used to my dick being there.
I pulled out and she coughed and spit up most of my cum. I was covered in her saliva, all over my stomach and thighs. Her chest was a sticky mess, mostly from the cum she was spitting up. I got off her chest, and she immediately pulled her panties off.
She spread her legs and wiped her hand on her chest, and then rubbed the sperm/saliva mixture into her pussy. It was a very erotic sight. I watched as she brought herself to orgasm in less than a minute. I couldn't take it any more and quickly moved my still-hard dick between her thighs.
Her eyes told me she wanted it bad, and I quickly thrust my dick up her pussy. She was so tight, I thought maybe she was a virgin, but I was too involved to notice if I broke her hymen. I sunk all the way into her, and she held me tightly against her body. We lay like that for a few minutes, my sister adjusting slightly to my girth. She writhed about gently, moving slightly one way or the other, until she cried out and came without me moving at all.
I fondled her tits and began humping her. I stuffed her pussy hard and fast, building our pace until we were frantically fucking each other. She began convulsing as another orgasm swept over her. I felt like I couldn't thrust deep enough or hard enough into her, I was overtaken by lust.
Out of the corner of my eye, I saw two people standing in the doorway. It was our aunt and uncle. Even though I was shocked that they were right there, my body didn't stop. I watched as they observed us fucking, and eventually I realized that they were fucking too. My uncle was pounding my aunt from behind. The thought of them getting off while watching us sent me over the edge, and I forced my dick as far up my sister's pussy as it would go, and gripped the bedsheets.
I came so hard I saw stars. I felt like I came ten times, spurt after spurt, each rope of sperm blasting her pussy with surprising strength. I heard my uncle cum, and my aunt with him. My sister was right behind us all. After we were all finished, she turned her head to see them standing there. They smiled at us, and just stood there for a few moments, and then went back to their bedroom. My aunt motioned for us to follow.
My sister and I stared at each other for a good minute, my dick still hard and buried inside of her. She rubbed her belly, where the tip of my dick was, and sighed contentedly.
Eventually we followed our aunt and uncle to their room, where my uncle fucked my sister and I fucked my aunt. That was the only time, however, and after that my sister and I occasionally had sex. Our aunt and uncle loved to watch us, however, and sometimes we would have a masturbation party. To this day I still ask my sister to wear her black bikini, and I'll fuck her sometimes while she's still wearing it.