I'm feeling completely powerless in this violent family situation, how do I make myself feel better about it?

You're mother has low confidence, exacerbated by being with this violent and controlling man. I think it would be better if she kicked him out of the apartment and showed that she has the backbone enough to get rid of him. If he is confronted by a a stong woman he will back down. If she shows a sign of weakness such as slipping out when he's not there to avoid confrontation and he sees her again he will be more likely to try and engage her. You can be there to support her. She should also get a restraining order against him so that if he does come back he can be arrested and prosecuted. If your mother just can't stand the idea of confronting him then wait until he is at work, put all his stuff outside and change the locks. Your mother will need a lot of support over the next few months as this guy has probably cut her off from her friends and broken down her self esteem so it seems that she is lucky to have him. Remind her that she is better off with out him, give her random compliments to boost her self esteem, and let her know how much you respect her for her decision and how much better off you all are.