The other day, we invited my cousins over to swim. Since it was pushing 100 degrees, they were here in no time. When we were done swimming for the day, I went up to my room to change into something dry, and I told everyone this so that no one would come in. Well, I went up and started changing. I was just putting my bra on when my 12-year-old cousin came barging into my room. He saw me without any clothes on!!! I told everybody not to come up, so he knew! What's worse is that he's done this many times since then, seeing me almost completely naked every time! He even watches me go to the bathroom through the keyhole! I've told my parents and his parents about it, and they don't believe me. They think that I'm just trying to get him in trouble. What should I do???