Provided you're sure all she's said to you is true and you want to follow a morally upstanding path:
Try to help her. Tell her you can't do stuff with her (Like kissing her and all) if she's going to hide it from her boyfriend and keep seeing him. Allowing her to cheat on someone, even if he's verbally abusive, isn't the best thing to do. If she likes both you and him, make it clear (But not in a harsh way) that she'll have to choose. If her boyfriend's verbally abusive, then it stands to reason that he's also not very receptive to her emotional needs/can't comfort her like he has to be able to. Which would explain why she came crying to you and not to him (Unless the problem originated with him, which is just further proof that she shouldn't be with him)... Of course, then there's the issue of your feelings. You've known her a long time and want to hurt her brother for hurting her. If your feelings are romantic, then do what I've said. If they're not, still try to help her get out of an abusive relationship, but don't lead her on into thinking that you feel that way. But considering your response to her, I think you want to be with her at least a little. Friends with benefits probably won't work here...
So try and help her. Make things clearer for her. And comfort her when she needs it. Believe that she's not using you, and if it turns out she is, then it's okay to be hurt. Have her choose, and if she can't choose, then give your own opinion. Even if it's "I want you to be with me and not him because he's hurting you with his words."
That's my take on the situation provided you want to follow the morally upstanding path, which I think you do, considering the title of this thread. However, you may have your own idea as to what you want to do in this, or you disagree with my viewpoint. That's okay.