It sounds like she may have a borderline personality traits and that your brother is codependent and suffering as a result of it. Is your brother going to be open to your feedback?
Not so much from me, but he sometimes is willing to listen to my mom.
If there is anything that he's willing to listen to, you may get together and let him know that you're concerned about his health and happiness. This way you focus on what he has control over versus his relationship. He has control only over his decision to remain with this woman for whatever reason of his.
His behavior serves him a purpose even if it is unhealthy. He could be insecure about himself, feels obligated or has dependency traits.
Ask him to tell you how does he sees his situation, is he satisfied, and generally what does he want. You are looking at the situation from his point of view even if you do not understand it. That is when you ask him to explain it to you and find out if he thinks that his life can get better. Women can too be abusive to their partners and he does not have to be in this situation unless he choses to for whatever reason.