Two wrongs don't make a right. There's a difference between being a punching bag and not being as bad as your sister. What I'm saying is treat her as you would like to be treated, even if she's horrible (feel like I'm quoting the bible here...), this is my mentality. If someone chopped off my legs I would forgive them (Maybe not in the moment, but eventually).
I personally am a Christian. I agree that it is important to always forgive, and I agree with the statement, "treat someone how you would want to be treated," even if the other person does not abide by that morale. If I were being abusive and cruel, I'd want someone to tell me, even if it makes me sad and/or upset for a while, because it is necessary to communicate that. If I were being selfish and didn't want to hear your explanation, I'd want the person to stop talking to me until I figured out what I was doing really was hurtful. Because I really do want to be a better person than I have been.
Forgiveness isn't always easy or instantaneous. Sometimes you really need to cut off or limit your time with a person to forgive them completely. Sometimes it is important to look after your own mental and physical health before someone else's, because in some cases, it is incredibly difficult to help someone when you aren't feeling nurtured yourself. Even if you already forgive her, just because you love someone and want to do the most for someone doesn't mean that it is the best choice for your own sake. Ultimately, you're going to be assessing what you think is the best choice. If you are able to find an adequate mediator, someone who will understand both sides and help you two work things out, whether a mutual friend or family member or counselor, that's great, even if you need someone there for safety and especially if she is cooperative! If not, then I'd also cut her out. Sometimes it takes separation for a person to realize what s/he's doing is as serious as it really is and to not take for granted the love given to him/her.
If you do choose to limit your contact to some degree or completely, you can still pray for her (if you do pray), hope the best for her, and be available when it's really important or when she decides she will work on putting away her abusive tendencies.