My mom beat her!!

PLEASE HELP US!! I need some serious legal help please! My mom is out of control. I don't care what my sister does, my mom has NO right to lay a hand on her. My sister is now with my grandma but I know my mom will try to get her to come back. I guess last night she beat my sister up AGAIN, and broke her phone. My sister has bruises and a goose egg on her head. My sister also has a brain tumor as it is and doesn't need this! She is 17. I want to make sure my mom NEVER gets her back! Yeah, my sister can be a shit and sassy, but usually it's because my mom is being a psycho bitch! I will be getting pictures of my sisters injuries and can personally testify that my mom has hit us for years! What else?!? Please help us!!