I had the same problem when I was younger, yet my sister was hanging around people she needed not to be. She was hanging around girls that well, we'll say.. were whores. She ended up almost getting raped by this Mexican guy that was 8 years older than her. I heard her and one of her friends talking about it and instantly told my dad. He asked her about it and she tried to denied it, with him not believing her he called the cops then she finally told me. The guy was prosecuted and sent to jail for 4 years. I was too young to handle the situation myself, wich in most cases is the proper thing to do, it gets the point proven and revenge from the other party is less likely. If you have a younger sister, or older sister with a boyfriend, ALWAYS pay attention. Remember, girls can get pregenant, boys can't. My little sister is now 11, and she just got her first period. Her childhood friend is a boy, he's 14 and just entered high school. Rest assured I keep my eye on him and have already had a talk with him. If you ever suspect someone hurting your family member it's best to go directly to the person that can help the situation the most, and the fastest. Always tell a trusted adult, or in some cases call the police yourself.
I hope I helped,
Precaution.