My friend and his sister
In the summer when I was 13 I went over to my best friend Steve's house. Nobody answered when I knocked so I just went in (our families knew each other very well). I went upstairs and he wasn't in his bedroom, then I heard something from his little sister's room (she was about 12). I went to the door and saw Steve's ass in the air between Mona's naked legs, and his cock rubbing her pussy. At first I thought she was crying but then I realized she was moaning with pleasure and she told him, "Put in in me, Steve, I won't cry again."
I was a little freaked out, being only 13, so I just went home and never mentioned it to Steve or anybody else.