That depends a great deal on what the local police are like. They may help. They may just let the family know about the complaint. Generations of abused women provide an unfortunate history of these issues.
And even if the police do help, they will not be able to provide protection. Their job is to arrest and prosecute the brother after he commits the murder. It’s sad and wrong, but that is the way the system works.
If you can get relatives to help, do that. If not, contact a local women’s shelter or other abuse-related non-profit. They may or may not be able to help directly, but they will most likely know what other resources are available.
I wish you the best.