I am sad

My grandmas birthday is on sunday and we all go to a Restaurant with the whole family and now it will get problematic because bf is still mad af at brother and bf doesn't want to come because we have to drive in the same car as brother and his girlfriend. And I don't know. It's again the old topic, that my parents will confront me and say "he doesn't show effort to be a part of the family" and my mom makes me responsible for a good mood on Sunday and I am stressed. I don't want to. I don't want ro be responsible. It's not in my space, I can't do anything about the behavior of my brother or bf. All I can do is to make a pretty face and don't make a scene myself. Everything else is not in my hands and doesn't belong in my hands. I don't want it. It's not my responsibility.
It stresses me out and I am sad. I don't know what to do. I don't want to be the one who tells my parents why bf isn't coming and I don't want to talk about that evening anymore. I don't want to be responsible for everyone. I can't do that anymore. I don't want ro and I won't. I tell my mom that she has to talk to them herself if she wants them to come and get along. It's not my responsibility. I won't do that.

Edit: I am also hurt. Because bf says now we are not together yet and I feel like he goes the easy way to just say no I don't go because we are no couple. I understand he needs time, that's ok but it Puts me under a lot of pressure and I don't want that. I can't handle that. I am setting between the chairs. I habe to puffer everything and I am drowning. I can't do that. It's hard to let that go.