So many unbelievable stories about siblings here. I will submit my TRUE incident that only happened once.
Since we were little kids my sister and I never even thought of doing anything sexual with each other, as many of the contributions posted here would suggest. We played together and fought frequently as brothers and sisters do. It wasn't until we were in our late teens did we begin really appreciating each other. I had never had one single sexual thought about my sister.
One day when we were 17 and 18, a junior and a senior in high school, we had a talk about dating and sex and all that stuff. In the conversation she asked me if I ever 'jacked off' to use the term she used. I told her that of course I did, and I asked her if she ever got herself off. She said she did now and then.
I guess usually the male is the aggressor and asks the nitty gritty questions, but my sister was wide open that day. Believe me, it surprised the Hell out of me when she said 'what do you think about masturbating with me, just at least one time?' I was pretty floored by that. She went on to say that she thought it would be interesting to do just once.
After asking her about 10 times if she was really serious and she kept insisting she was I thought about it and agreed it could be interesting, so I said OK. She smiled and said 'where should we do it?' I didn't care because we had the house to ourselves for the afternoon, so we went to my bedroom.
I said 'we gotta get naked you know,' and she said OK. Slowly, almost tentatively, we undressed in front of each other until we were both naked. If I wasn't getting hard already, seeing my sister in the buff sure finished making me hard. Her 17 year old body was very nice. I hadn't seen her naked probably since we were 5 and 6 years old.
She had at least 34C cups and brown nipples, and her pussy had a big mound of dark brown hair growing on it. She let me feel her boobs, and she let me feel her pussy for about 10 minutes, and she felt and squeezed my throbbing hard dick. She asked me how I jacked off, laying down or sitting, and I said both ways. She said she always laid down. She laid on the bed and I asked her how she did it, so she started rubbing her pussy and clit. I sat on a chair beside the bed and watched and started stroking my dick. She asked me some questions about stroking speed and things like that. We fell into silence as we watched each other. I usually cum in a tissue when I'm masturbating, but she said she wanted to watch it spurt, so when I couldn't stand it anymore I spurted all over my stomach. She watched closely when that happened and asked me questions about how it felt to cum. Then she closed her eyes and in about another 5 minutes she came.
When we were done, she acted a little embarrassed, but she thanked me for allowing it to happen. We never did it again, and I can't remember ever even talking to her about it again. It was a one time only event.