This happened at one of my friends parties. It was a slumber party and it was me, Amy (the birthday girl) and Nicole.
It was the middle of the night, and I had to pee, so I went to the bathroom upstairs. I went to get something from Amy's room (we were sleeping in the basement) and saw that the light was on. I peeked in and saw that Amy's brother was standing over Nicole's sleeping bag looking at her underwear and masturbating. I noticed that her underwear were hard in some places and that didn't surprise me, she always kept her underwear on when she masturbated. Amy's brother's penis was long. He was 12 or 13 years old, I think.
As I watched him masturbate, sometimes licking the hard part of Nicole's panties, I felt myself getting wet. So I stuck my hand in my pants and started to slowly push it in and out of my pussy. Pretty soon I took all of my clothes off and I was pushing my hand in and pulling it out at the same time as he was pumping his dick. I had a huge orgasm and he heard it and walked out of the room. I told him that I'd been watching and asked if I could finish him off. He said yes and we had a fun night. I hope this story makes you as horny as it makes me. I still masturbate while thinking of how his dick felt. Happy Jilling!