Senior Getting to friendly with Freshman?

I'm a junior in high school that has a problem with a senior guy hitting on a freshman girl that is like a little sister to me. It started over the summer when she met him and he just out of no where started to go after her like he did a freshman last year. He doesn't have a good reputation with girls and a lot of people knows whats happening. I don't know what to do know because every time i warn her or try to get someone to just keep an eye out she thinks im starting drama and **** like that. I mean she will ask me what to do about him hitting on her but she doesn't listen. Her sister isn't doing anything. What should i do?