My friend was in a (very) similar situation last year, and here's what I did for her:
1) I was always there if she needed to talk about it. I never pushed her to talk, but when she needed support she knew she could come to me. When she wanted to talk, I made time for her and listened to her. I'd assure her it was going to be okay.
2) I talked to her about telling the police or somebody who can get help for her because in her situation, she was beat up because her father (who was a teacher at her school) thought she was telling people. Until she was ready, I never told anybody, She came to me because she trusted me and I did not break her trust. It's your friend's choice whether she's ready to tell anybody, but maybe you could suggest that she talk to her parents or another trusted adult about it when she is ready.
3) We set up a safety plan, if she felt threatened and could not call the police she could text me a coded message and I'd call for her. If she needed somewhere to go, (with permission from my parents, although I didn't tell them the situation) I told her where our extra key was.
4) When my friend was ready to tell somebody, she asked me to go with her because she felt safer with me there. So I immediately went with her.
That's just how I handled it, but no two situations are the same and so what works for one might not work for another. In any case, be supportive. Maybe set up a similar type of plan, in case her brother does try to beat up her or her brothers. For example, if you both have cell phones, you could both come up with a phrase that she could text that meant she needs help. My friend and I came up with "Beth's calling" it was a phrase we wouldn't ever use in normal conversation (we didn't know any Beths), but wasn't weird enough to raise suspicion.