Ahhh...I read it to be more medical, but surely tolerating abuse becomes both physical and is about self esteem and other emotional issues, too!
IT's hard to understand from the outside, Shell, how people can start to believe they're worth so little, that they deserve to be treated that way.
It's hard too for me as a guy to understand how anybody gets their jollies whackin around a female...but there are lots of ways to abuse: mental, emotional, verbal... and both genders can and do them well!
I have gone through this with dear friends... the key, as you said, is to get her to see her own value and that God loves her and He "don't make junk."
You're rage is also understandable...but we're called to hate the behavior and love the offender. That one still escapes me in situations like this.
Prayers for your sister to be touched by the Holy Spirit to help her see the pickle she's in and realize that "only she can prevent forest fires"---and as overwhelming as that must seem to her where she is, God can make it happen if she but asks... in His time...and that is the hard part!
Peace and prayers for all...