My friend is getting majorly bullied!?

My best friend has a 13 year old stepbrother. Her parents are divorced, she is with the dad, and the dad's girlfriend and son (the stepbrother) have been all living together for 10 years. The brother always causes trouble and repeatedly abuses his 11 year old stepsister, and get this! the sister is the one who always gets punished! She tries to tell her parents about her brother, but they just don't listen or pay attention! Like yesterday, she got kicked twice and has two bruises on her shin and knee and she is the one who gets grounded!!! What should I help her do or what should she do? She's been trying to tell someone for years and they don't listen!!!!!!! Her mom is the only one who listens and she lives far away!