This happened once with my sister, and I'm still amazed it did.
This is a completely true story. At the time of this I was 18 and my sister was 16. Having grown up together we were typical brother and sister. We didn't fight a lot, but had arguments sometimes. We generally got along though. I was fairly small and skinny, but had a decent amount of pubic hair and had a decent size penis. My sister was a little chubby, but had nice round breasts and plump butt. On accident sometimes we would catch each other coming out of the shower naked or half-clothed, but never anything much and it was always just a brief glimpse if that.
One Saturday evening our parents decided to go out to eat for their anniversary. My sister would usually hang out with friends in the evening, but this night she didn't. So the two of us were left at home. I spent most of the evening in my room on the computer while she was either on the downstairs computer or watching television. While up in my room I closed the door and, being a horny teenager all alone, decided to find some porn to watch. I surfed around a bit while having my pants down slighty with my erect penis sticking straight up.
While watching a video and really getting into it my sister suddenly flung open the door to say something when she saw me, penis in hand, jerking away. I quickly pulled my pants back up and told her to leave me alone. I was pretty upset that she had walked in on me, but she just kind of stood there, not sure really what to do.
Seeing that she hadn't left yet I started to get out my chair to shoo her out when she says 'what were you doing?' I quickly responded 'nothing, just go' while still moving towards the doorway. She replied 'do you do it often?' I slowed down a bit but still tried to get her to leave. 'Never mind, just leave' I said again to her, this time standing in front of her at the door.
At this point I was fairly surprised with how open she was, as we had never talked about anything sexual like this together before. She then stepped back a bit and said 'I don't mind, I mean...you know...it's ok with me.' Now I was shocked. Was she saying she liked it? I still wanted her to leave though as I felt a little awkward. She surprised me further next when she said 'If you want, we can, well...like...do it together.'
Now I was floored. I didn't know what to think. What I did know was that I was getting more excited and less upset with my sister. Feeling that this was still awkward I said to her 'No, just go, it's a private thing.' To that she replied 'well yeah, but we both do it, and if we both are ok with it then it's not a big deal.'
She was making sense in my head, and deep down I thought it would be hot and exciting to masturbate with my sister. But I still wasn't fully onboard and had my reservation.
'I won't tell or anything, you know? And it's not like we're having sex.' Before I could even respond to that she stepped back into my room and headed towards my bed. She sat down on the side of it. 'You can keep going with your video, I don't care. I'll just be on the bed.' Suddenly she had taken charge. This whole time I was left speechless as she was now on my bed ready to masturbate alongside me. I quickly shut the door behind me, afraid now that our parents would come home and see what we were about to do. I then slowly approached my computer and sat down. 'Just stay on the bed though ok?' I said to her, still nervous to go through with it but not wanting to stop. She just mumbled 'mmhmm'. I then started my video back up, and slowly pulled my pants down, exposing my still hard penis. I turned around slightly to see if my sister was watching when I saw her on her back, breasts exposed, taking her panties off. I got a great view of her thick bush and her gorgeous tits. I got even harder. She then quickly looked up at me, while on her back and said 'ok I'm ready'
We both quickly started masturbating, her on her back fingering herself while I was jacking away. Instead of watching the video I found my eyes fixed on my sisters pussy, her fingers working her clit. Her other free hand would ocassionally rub one of her nipples. I could tell her pussy was very wet and could hear her fingers sliding furiously in and out of her. The whole time you could hear my hand beating my extremely hard cock, the head bright red with precum dripping out of the tip. I was nearing orgasm as I stared at her wet pussy and her hard nipples pointing up.
As her moaning got louder my stroking got faster. In the middle of it all while catching her breath she shouted out 'tell me when you're cumming' Without time to react I said 'I am now' She popped her head back up to look at me while still rubbing her pussy. With both of our eyes fixed on each other I started to shoot my load, squirting all over my hands and chest, some hitting the floor. She grinned slightly at me while she kept working her pussy. Then her head fell back and she moaned really loudly, saying 'ok, ok!' Suddenly her back arched, her toes curled, and she started panting some. I kept stroking, getting every last drop of cum out of me. I could tell she was cumming, and I stared at her dark, wet pussy with her perfect round butt in the air. She then lowered herself back down, still panting and laying on my bed.
After we both sat for a few minutes just catching our breath she sat back up, looked down at me and said 'ok, I guess we should clean up', giving me a smile. I just smiled back and nodded my head. I was in shock with what just happened and didn't know what to do. She got up first, her round butt shaking and her tits bouncing. She reached down and put her clothes back on. I then got up to get a towel and clean up. She left just walking away and said 'thanks, that was nice.' I didn't say anything but just stared as she left.
We both went about our night and nothing else happened. This is the only time this happened, but I obviously think back to it, still in awe to this day that it even happened.