This is way easy, stop talking to your friend and his GF. There's too much baggage here.
Your friends with a guy that beats his GF and abuses her, and your going out with her twin sister. First off WHY in the world are you still talking to this guy? If I knew that someone was beating someone else, I'd be the first to call the cops.
With his GF you have to understand that this girl has very low self esteem, mainly because she's being abused. She will not be the one to break up with her BF...and she's very clingy at this point too. So she's going to find the nearest source to give her the attention she isn't getting from the BF...which is you! Any woman who has been mentally and physically abused needed counseling...period!
If she confronts you and wants some sort of explanation (which she probably will), tell her that "she still has a BF and is giving you way too many mixed signals" and you don't want ANY part of her or your friend anymore. I would tell her that she needs help. You personally have become the "dumping ground" for the both of them, by listening to his problems and for her to be all over you. Maybe you are getting bored with your GF too. I think the reason you are falling for this girl is because you feel the need to "rescue" her in someway and know you can take better care of her.
Bottom line......stay away from the other girl and your friend. Make yourself "unavailable", change your number or do whatever to stay away from the both of them. Even if you have to tell her what her BF is doing behind the scenes, or you have said to her sister that she's being abused, or that you are tired of being the dumpster and your done with her and with your best friend.
She comes with WAY too much baggage, and baggage is always going to be a problem regardless.