When I was 13 or 14 I had a hardon most of the time and my sister and I slept in the same room. The only thing separating us was a wooden divider. At night I would masterbate every night and my sister would watch me. It was very exciting knowing she was there and I would just explode when I came. Other times I would watch her play with her tits and stroke her pussy. We never did it together. Still get hard thinking about that.