What should a person do with destructive family members and their enablers?

One of my brothers, diagnosed with a mental illness, is causing serious problems for my parents. My parents invited my brother to live with them. This brother is 26 years old, he does not hold a steady job, and he has been emotionally and physically violent with my parents. At this point, my parents will not kick him out for fear of being hurt. I don't live at home, but I'm deeply worried for my parents. What should I do?