This happened two years ago. My friend and I were 16 (or maybe 15). My sister would have been 14.
My sister and I are the only children of a standard Christian English couple. We live near London. My good friend from school is a guy named Ben. He's the only Jewish guy in the school and my only Jewish friend. Living in England, that made him the only circumcised friend of mine. I had never seen his dick before this incident but I obviously knew he was cut.
My sister, Ben, and I got along well and always joked about sex and wanking. She knew that Ben and I wanked, and we both did it quite a lot just not with each other. I had done it with a couple other mates but not him. I had actually wanked for my sister before but that's a different story.
One day we were joking around and I said 'hey Ben lets go wank it in my room.' My sister, Lizzy, said 'No stay here. I like to watch. And I've never seen a Jewish dick.' That last part made me crack up and Ben go really red. It actually sounded like fun to me so I whipped my uncut dick out quick and started wanking. Both of them just stared for a minute and then I told Ben not to be a prude and to join us. He resisted but I convinced him. When he took out his cock Lizzy just stared at it like it was from Mars as it grew to its full length (which was quite long but I'm not good at guessing). We both came really fast and Lizzy loved it. Ben left soon after and Lizzy and I had more fun. I came again and she jilled herself off for the first time in front of me.
After that we began a circle jerk. We would masturbate together all the time. Eventually Lizzy made a move on Ben's cut cock and started jacking him. They became a couple by the time he was 17 (still together). I got to see them do everything. I've seen his big cut dick cum all over her (and in her).
It's been great.