You are way, way too interested in your brother's situation. Whether to stay with this young women, or not, is totally his business. Stay out of it.

Yes, when she called, it would have been best to say little. You can't possibly advise your brother or his girlfriend. You basically disapprove of this relationship, but you need to let these two figure things out for themselves. She called and sought you out as an ally because she obviously doesn't realise you think your brother ought to end it with her.

All you can say is, "I'm sorry your going through a hard time." Then leave it alone. Her being bipolar hasn't got much to do with anything. I'ld give you the same advice, if she weren't.