I don't think that one can underestimate what sustained emotional abuse does to you. I think people who imply she is the one in the wrong, or that she is just as bad as him, don't really have a grasp on what emotional abuse can do to you. It sends you mad. You respond irrationally. I see their need to not condone physical violence, I completely understand it, but sorry, I do think people can be driven to it.
I think physical violence is bad, yes, and I cannot condone it. But like I said, emotional abuse can make you lose your mind, go irrational. And I would say this if the gender situation were reversed. I don't think we should accept violence of any kind. But tour sister is a victim, and it sounds like this is the act of someone at the end of her tether.
I was emotionally and verbally abused for years. I am a non-violent person, a non-angry person, I despair at violence, I can't even eat meat because of the violence it represents. But during my time with this abuser, there were moments when I felt so trapped, so insane, so maddened by the constant sense that I was useless, pathetic, pointless, disgusting, worthy of contempt, by his sniggering, lip-curling mockery and despicable, loathsome words, that I wanted to do violence. I did it to myself, to my walls, to the furniture. I managed to avoid doing it to him. But I think I probably came close. It's a madness, a mania.
And while, like I said, one can't condone it, one ought to be able to understand it and make allowances for it if she's in this situation.
If you can, keep talking to her. It's hard to hear the truth when you are in that situation, but perhaps it will make a difference. Ultimately she has to make the decision to leave herself.
Sorry this isn't more helpful.