My girlfriend is being ually abused and I don't know what to do?

My girlfriend (Tina) is being ually and physically abused by her older sister's(Sarah) husband.We are both currently in the 11th grade and have been going out since the 9th. Her parents are strict and barely lets her leave the home, except to go to her two sisters home one being a safe place (Melissa"s house) and the other being the place she gets abused (Sarah's house). She just told me this about a week ago and told me its been happening since summer 2010. He has never d her but he has touched her and beaten her. The last time he forced her to give him while he was driving her back to her house from his home(Sarah's House). He never does these things while her sister is home, but every time the sister leaves and Tina tries to go with her, he will try to make her stay to watch their young daughter who is a toddler. He has also verbally abused her while he has molested her. I want my girlfriend to tell but she thinks it wouldn't be the best thing for her entire family. Sarah is currently in love with the guy and thinks he can't do anything wrong. Melissa is currently going through a fight with cancer and is sick. Her dad is very stubborn and strict (hates me btw) and she believes he would be devastated to find out the one place he feels she is safe outside of his home, she faces her biggest problems. I don't know what to do in this situation. I know people will say call the police but at the moment I don't think that is truly the best solution. Even though this guy is a molester he is a good father and personally coming from a home with a single-parent I'd hate to be the person to have someone else grow up fatherless. Tina currently feels guilty and thinks that its her fault that these problems are going on. I seriously want to help end this because I have strong feelings for her and I believe in no way shape or form this behavior is acceptable. I had a plan to threaten the guy and tell him to stop in a facebook message but the more I think about it the more I think this is putting Tina in harms way. Tina says that the guy tells her he feels guilty and will stop but I take this as total BULLSH*T. I'm not a specialist in this but I know for sure you can't count on his word in this situation, especially when he keeps doing it. I told Tina that if she doesn't handle the situation I'll take matters into my own hands. The guy lives close to my older brother and I honestly would love to get my hands on him, but I know that violence isn't the best answer in this situation. Because of the damage this has done to her emotionally it causes issues in our relationship. Please help me come up with a solution