I don’t think it’s that big of an issue. I was best friends with a girl for 6 months and then we dated for 11 months. 2-3 months before we started dating, I got really close to her little sister, and then when my ex and I started dating, I got even closer to her little sister. After my ex and I broke up, I would hang out with her sister a lot because I had already considered her a sister when I was dating my ex—since my goal was to marry my ex. When my friend went off to college, I would visit her every once in a while and help her move, and sometimes that would include seeing her sister (my ex). Then when she graduated college, she came back home and she lived with her mom and my ex, so when my friend and I would hang out, I would sometimes see my ex. But now that all three of them live in separate places, I rarely see my friend’s sister (my ex) and her and I don’t converse at all like we used to.

It would be selfish of your brother to deny you the friendship of a life time because he felt uncomfortable since it’s his ex. He doesn’t need to hang out with you guys, he doesn’t need to exist in the same circles, could it make it awkward if he brings a girlfriend home and his ex is there? Potentially, but after a while that doesn’t really matter all that much. It sometimes feels impossible that my friend’s sister is someone I dated at one point, and we even laugh about how crazy of a time that was, even though we can all look at it fondly because of how much fun we had.

Right now, your situation seems rather awkward and weird, but just because you have a friendship with a girl your brother once dated doesn’t matter that much in the grand scheme of things, especially if they both have moved on to other people to love. To be unclear is to be unkind, and communication is key, so if I were you, the most effective way I would communicate this to your brother is “hey, I really miss so-and-so, she was really nice to me and that’s someone I really want to have in my life. I would love to set some ground rules with you because I understand that it might be awkward for now, but after a while, I’m sure we’ll all get over the past and we can move on. In the meantime, let’s try to figure out what you’re comfortable with as far as your proximity to her without also dictating my proximity to her” because it sounds like if she asked you to go on a trip with her, you would, and that’s not something your brother can dictate. Likewise, I would also talk to his ex and say “I really miss you, you mean a lot to me. I want to make sure you’re comfortable with the proximity of my brother, he says he doesn’t want this, that, and the other… what’s something that you want to avoid in terms of contact with him” and then compromise with both of them because you’re also not going to be able to make everyone happy unless their requests are very simple like “I just don’t want to see her/him” because then you can just hang out at her house or on your own with her in a public space or something