Put yourself in the place of a Mom whose child comes home with a boy/girlfriend you do not like. What's your best course of action?
Answer: Polite Neutrality.
How would you feel if your mother was trying to break up you and your guy?
If you actively try to break them up, that might push them closer together. "I'll show her she's wrong."
It's a lose-lose situation. If they eventually break up, they blame you. If they stay together, then you look bad, and you've got a long-term bad situation.
However, if you stay neutral and welcome the new girlfriend politely, then it's win-win for you. If they eventually break up, then no one blames you. If they stay together, then you can build on neutrality.
My suggestion to you is to let the relationship play its course. If she's really that bad, Brother will figure it out for himself. Let him. In the mean time, just be as nice as possible, as you would be nice to anyone. That's not hypocritical. You are keeping your Private Thoughts to yourself while your Public Actions are courteous and polite.