I never understood why I dreaded family gatherings until I learned about BPD.

I can understand why you're nervous about your brother's wedding--what you've said that will likely happen does sound very unpleasant.
How long is your mother going to be away for the wedding? If it's only for a few days, you might be able to minimize the impact on your relatives (and therefore yourself) by putting her up in a hotel or having her spend a night "visiting" with each willing relative/friend If it's longer than, say, a week, it might be worth reaching out to your relatives/friends to see how long they could put her up.
Does she have a direct flight? Even if it's more expensive, that might be worth looking into just to make sure she doesn't miss a connection and end up stranded overnight.
The good thing is that you already have an idea of how your mother will behave. You also have some time to set up some plans if/when she starts to behave the way you've seen before so you can have some help when you need it or a way to minimize the disruption.
You're not crazy for being anxious about what's coming up. Many of us here can relate to what you're feeling.