I think there is something seriously wrong with my 9 year old brother?

Okay, so, I am 13 years old, and I have a little brother who's 9. Lately his behavior has become really disturbing. He's constantly making sexual hand motions at me from accross the room, and denies it when I tell my dad. I have no idea what the hell to do. I don't know what his problem is. I've lived with my parents for my whole life (I'm adopted.) and my parents adopted him and my other brother when they were about 1 and a half. Has this adoption thing messed him up? What should I do? I'm really scared, and those hand motions are really frightening. I'm afraid he'll act on it. He's almost my size already! HELP!