I just discovered my brother (29M) has secretly been seeing my college ex-girlfriend (32M) over the past year. We broke up 8 years ago and we’d been going out 4 years prior to the split but we actually knew each other very seriously when we’d been together.

I challenged him when I discovered this and he said he didn’t say anything to me b/c he didn’t want me to get angry with him.

Last weekend we celebrated my parents’ 40th anniversary celebration. My brother brought her to the party as his guest without letting me know beforehand.

Her arrival with my brother left me shocked since we broke up and I hadn’t laid eyes on her since then. During the meal, they declared their engagement and my mom began crying tears of joy.

I couldn’t take it and left. My brother trailed behind me and we got in a big fight. I told him he should have warned me at the very least. He said I was being self-centered and spoiling our parents’ party.

That evening my dad phoned me telling me I humiliated the family by leaving.

I don’t have romantic feelings towards my ex anymore, but the surprise reveal and secrecy at my parents’ celebration feel thoughtless. AITA?