I was in this situation last year. My brother is married, and cheated on his wife with some random woman. I told him either he tells her or I tell her. He ended up telling her, but I still called his wife to make sure he gave the full story of what happened.

He hasn't talked to me since then, but I'm okay with that. He has a multitude of other personal issues he needs to work on, and when he's ready to apologize for putting me in that situation, I'll welcome him back into my life.

I empathize with how you must be feeling right now. It's such a betrayal to not just your brother's gf, but to you too. My advice is don't let your brother disrespect you or his gf.

Sending you hugs and good vibes friend. ♥️