From one mum to another, there's nothing quite like an intensely loving mum who feels so deeply for her kids. As a ferocious lioness, when triggered, and as solid guide and advocate for her kids, watch out anyone who gets in the way. Wondering whether the people around you are appearing somewhat insane at the moment, based on their actions, inaction and reactions. Wondering whether 'What the hell is wrong with the people around me?!' is something that's entered your mind.
With this involving repeat behaviour, when it comes to your partner's brother, I believe the man's mother really needs to step in and address that issue with her son. What the heck is she thinking? Where else has he been repeating this behaviour? Is this simply unacceptable behaviour (where you have every right to reject both it and him) or is it criminal behaviour that needs addressing? Does he have a deeply disturbing issue which can lead others to some form of trauma? Is there a need to take matters further? No need to answer, just throwing some questions out there.
Based on what you say, the mother sounds self serving. Wondering whether she's accustomed to having people serve her, as opposed to them challenging her on the odd occasion. You may be her first serious challenge and you'll probably receive a few choice labels for that. You may have to wear them with pride. Now, I admit I'm being incredibly childish here as I sit with a grin on my face but would be tempting to say to her 'We're having another baby and I've decided to name this one after you. We're calling the baby 'Unreasonable woman'. As I say, very childish but I wanted to put it out there just to get a smile out of you. In such a highly angering situation, it's important to feel something other than anger.
Personally, I find anger or fury to be highly motivating and revealing. Being careful with how I manage such emotions, I always rely on them to be 'telling' emotions. They'll tell me what I won't settle for, what I won't tolerate. Fury is a definite boundary setter, when it comes to telling emotions. It will also lead me to be more reasonable or reason able. 'Am I able to see good reason for my intense feelings? Do I trust those feelings, even when others can't relate to them or don't agree with my perception?' Fury can be a thoroughly reasonable emotion at times.
'Grandmother' is a title that should be earned, not automatically given. Behaving in a grand magnificent way requires consciousness.