Divorce and child Custody

Hello. I'm asking this question for my mother. My parents are married for 25 years. I'm 21 years old daughter and I have 16 years old younger brother. Since childhood, I have seen all the physical and mental abuse done by my father. He has been very cruel with my mother as well as me and my brother too. She has never filed a complaint against him. But I can't see this anymore. I am convincing her to go for a divorce but she is afraid that she won't get the custody for both of us. I am well educated, unmarried and employed. But she is unemployed and less educated.I can take care of her with my earnings. With a time being, I'll get her started with some business. But right now I won't be able to spend for my brothers education. And she doesn't want to leave him with my father. Also, she has a fear that my father will try to harm me or her for taking this step. I don't understand what to do. Can my mother get the custody for both of us? And can we get police protection for some period so that he won't harm us?