She must enjoy it in her own way. I'm sure she'd rather not get hit, but the whole scenario is a habit and feels comfortable. You say they don't talk except for when they are drinking. This way, at least they do talk a bit, even if it's abusively, and she gets attention of some kind.
I'm not defending your stepdad at all. He is behaving appallingly but if she lets him behave like that he will carry on. Perhaps you should be talking to him about it instead of your mum. Can you do that? How is he with you and your sister when he has been drinking and when he is sober?