In-laws :0(
Life's too short.
I am a very calm and non-confrontational person. I hate to argue!
But..
...ffs... they have been rude!! Would they treat their friends in that way?
Sometimes families are the nastiest, most spiteful people to be involved with simply because they know they can get away with it.
My advice would be to NOT go to your mother-in-law's do. To tell her, and anyone else the decision may affect, beforehand, and then have a bloody good time doing what you WANT to do, instead.
So what if it causes an argument...? I mean, really...? There are worse things to happen to a family.
Please yourself and feel good about it!