Hi there,
Based on the tone of your post, I'd say you want to support your sibling and you should try to do that. Now, you can't control what your mom does, but if you think she wants the same as you (i.e. to support your sibling), then it's possible that she could keep her act together to achieve that goal. You're right, getting into the various reasons for going no-contact isn't appropriate at a family event like the one you describe . . . and my hope would be that everyone would decide to be the "bigger person" and put their differences aside for the duration of the event, while treating everyone with civility and respect. If you think that's possible, then I'd say, you go support your sibling.
Other possibilities here would be to talk to your sibling, and say something like, "I really want to support you at your big event, but I'm fearful that issues between me and Mom could detract from that. I'm prepared to do whatever you think is best." Maybe that means, coming to the event and hanging close to a neutral person as a "buffer," or possibly "popping in" for a short visit to minimize direct contact with your Mom, or maybe (if there are seating assignments), working out with your brother a low-contact seating arrangement. In the most dramatic case, maybe that means your brother understands that you're going to decline to participate in the big event, but that you show your support in another way, say at a side-event for the siblings only. The big caveat here is whether your mom has enlisted your brother as an ally on "her" side, by inventing stories of victimization and ill treatment by you, creating a potentially toxic triangle. If you think that's the case, then I wouldn't enter the triangle at all and wouldn't consult the brother about your concerns.
Notwendy brings up a good point, which is that some pwBPD can "hold themselves together" in front of people they want to impress. If your mother is like that, and if the event is a situation where she wants to impress others, then I think you take the risk. If she's like that, then I think you give her a quick hug, say "It's nice to see you," and then quickly re-focus the conversation on your sibling ("This is wonderful, I'm so happy for him.") Then make a little excuse ("Oh, I've got to go say hello to cousin/I've got to freshen up, I'll catch up with you later") and then make your exit.
Look, I'm not sure how old you are, but when I was a young adult, there were times when I didn't contact my parents for weeks or months . . . not because I went LC, but just because I was busy and needed to break the "child" routine! Initially, I'm sure my mom was "hurt" by that, because she really wanted ("needed") to talk to me every day, as she was somewhat emotionally clingy. But eventually, we established a more "reasonable" amount of contact (for me at least), with a half-hour call, two or three times a month. But before getting there, I had to break her of the habit of calling me all the time (e.g. at 7 am on Saturdays and during working hours). That's a long-winded way of saying, I think that going NC or LC with parents in early adulthood is normal, because you have your adult life to live! While not talking to your mom since August is a lengthy break, it's not unheard of, and it could be completely justified ("I've been really tied up with work and studies, I didn't realize it has been that long"). I guess that's why on these boards, when it comes to NC or LC, I generally recommend sending pwBPD a short greeting on critical days, such as Christmas or a birthday, just as a civil and kind gesture of acknowledgement, provided that you're feeling up for it.
I'll close with another idea for you regarding logistics. I've found that when visiting with difficult family members, it's best if I arrange for my own hotel room and transportation. I'll usually decline invitations from a host with a reply like, "You've already got a lot on your plate / I don't want to impose / I've got hotel points that are about to expire," etc. If I have my own hotel room and car/Uber, then I'm not reliant on anyone else, and if I need space, I can take it. By the same token, pwBPD tend to feel overwhelmed by the stress of events like the one you describe, so if they can retreat to their own quarters, without having to worry about YOUR meals/lodging/transportation, that takes some stress out of the situation. I'd say, even though it's a little expensive, you go ahead and book your own hotel room and rental car. Think of it as emotional insurance for a potentially stressful event.
Just my two cents.



