Can I ask why you don't like the idea of your friend dating your brother
ikr in an outsider's pov i feel like they didn't do anything wrong other than keep it a secret from you but can u please explain as to why you're so against it?
maybe op not hating the idea of their friend having a relationship with their brother, but the problem is that they're keeping it a secret from them. like y'know, there are some times where you believe so much in people to the point you want to tell them anything about you and want them to tell you anything about them. no single secret.
but for op, i think that you have to talk it out with them two. there must be a reason why they keep it a secret from you, not that they can't trust you but any other particular reasons. i don't think it's wrong to feel kinda upset about it, but not forgiving them and not giving them chance to explain isn't good either.
I have a best friend of 9 years old. last year I found out that she having a relationship with my older brother behind my back and I found out when I see them in my brother's room one day when I came back from school. I really feel betrayed by them. And I never spoke to them ever again. Recently my brother just texts me and said that my friend really feels guilty and I still can't forgive them because my friend really knows that I have trust issues and to think that she and my brother would break my trust.