You ignore your mother and focus on your sister. I don't mean that harshly, but your sister has a child. This time, she called the police. She may not be able to next time. He knows that move now. Make sure she has somewhere to run at any time of day or night if she needs to. And very, very gently, tell her that this is what love will look like to her daughter if someone doesn't break the pattern. I am so sorry this is happening. Been there, done that.