I found out what happened last night with my girlfriend and I don't know how to handle this?

So, I found out that what happened last night was much worse than I anticipated. Apparently, her dad, being insensitive again, said something, I guess, wrong. Lexi must have said something that upset him greatly because he slapped her, which he hasn't done since she was 7. Her newly-found brother, intervened and began fighting with her dad, and left after being insulted and slightly beaten by her dad. I really have no clue how to handle this. I'm only 16 so I know I don't have the right to go to her dad and risk making matters worse, and I have no clue on to her brother's whereabouts, and I am not leaving Lexi alone by herself. I've already told my mom but she really doesn't know to do either. Lexi forbids me from going to the cops. I want her to stay here with me, but she would need to get her clothes, phone, id cards, etc and I am NOT sending her back into that house without some sort of protection. So, Yahoo Answers, what do I do and how can I help my girlfriend with this problem?