My brother wants to bring his mother-in-law on our 'siblings only' vacation and I'm thinking of cancelling.
Do you have an issue with your brother's mother-in-law?
I really just hate my brother's mom-in law.
Cancelling your siblings' vacation just because you have issues with your brother's mother-in-law is not a good idea. Hating her is even worse. Find a way to get to know her and you will find out how to love her instead. And if you don't love her, at least you'll be able to tolerate her a bit more.
Shut your mouth! I need to get away from that bitch.
I get your frustration as you're not going to like every family member, but cancelling your trip seems extreme. Your siblings would miss your presence and if you can use this as an opportunity, to improve your relationship.