During the summer when I was 16 I often went to my girlfriends at the time's house in the daytime when her parents were at work an they left my 14 yr old girlfriend home with her 10 yr old sister. One day I was on top of my girl fucking her brains out when her sister walked in out of curiosity and caught us fucking just stood there in awe. MY girl initially got upset but I told her that her sister just wanted to learn. After that her sister watched us fuck on many occasions.