 1st off, I would appreciate it if no one suggested I dropped my friend group as we have a very long history. I'm more concerned with my best friend/brother.


Anyway. I do not know how to make healthy boundaries. I've always bent over backwards for people but specifically my family and my brother because otherwise, huge fucking blowout. 
Brother is bipolar and obvs, I BPD. 

We are supposed to go to a friends bday party at their house tonight (15 ppl tops but currently the city has closed restaurants and bars due to Covid) but my boyfriend is worried and doesnt want me to go as his mother is elderly and sick and we live together. I'm also a bit immunocompromised but I'm terrified of upsetting anyone. I've been isolating myself for so long and I'm scared of them thinking a certain way and pissing off my brother. 
I dont feel like I'm allowed to have feelings. But I cant talk about them without being screamed and gaslighted. 
This is a very small part of him, otherwise beautiful man but holy fuck I cant handle it. 

What do I do?