Problem is that if you inform somone that goes straight confrontational with the aunt (such as the authorities), she will very well deny it all and then blame it on your friend for telling lies. Your friend who is very scared of the repercussions may very well side with your aunt in a bid to protect herself which will throw any chance of anything getting done.
I'd say, inform the brother/the right cousins, but make sure they don't act without confirming it through the friend FIRST. That way, they at least have heard it from her before the aunt has chance to pollute the answer (and oh will she try).
The key will be getting your friend onboard to try and help herself out of this situation though. She's in a frightening position and you need to support as much as possible so that she can have the courage to do something about it.
If you eventually escalate things to the police, you should probably get her to have concrete evidence of what your aunt has been doing before doing so (maybe pictures of bruises?). As **** as it is, hearsay cases get dismissed a lot by the police/authorities, even when it involves abuse of minors. You do have to tread lightly though, the wrong move can put her in more danger and/or restrict her ability to communicate with anyone that can help, unfortunately.
Good luck.