Something similar happened to my best friend and I. We were both 16 at the time (22 now) and his step sister was 18. He told me that his step sister was starting to walk around the house with only a T-shirt on when his parent weren't home. She later started to tease him and would ask him if she could join him in the bathroom. My friend asked me if I could sleep over that night because he was starting to worry. We slept in the same room so I would have his back if his step sister came. I woke up to moaning in the middle of the night. I saw his step sister on top of him in the nude and I tackled her to the ground. I check if my friend was ok and he just pushed me and started -blam!-ing his sister again. He gave in and so did I later in the night. Oops.