Custody when parents aren't married...?

I'll keep this simple:

My sister-in-law is 21 years old and has two small children with an abusive bum. She's been with him since she was 14 (he's 30) and her daughters are 2 and 3. He has anger management issues (cops have been called in the past) and he hasn't worked since the oldest daughter was 3 months old. She goes to school and works almost 40 hours a week. Thankfully, she's finally made the choice to leave him and move back in with her mom while she finishes school.

Here's my question: how does she get full custody of her daughters? He watches them while she works, but I worry that he'll ask her for child support when she finally gets through school and gets a job. She'd be willing to have someone else watch them during the day, but she still wants to allow them to have some contact with him. What should I advise her to do? (She asked me.)