Tbh your updates make me more sympathetic towards her, which I didn’t see coming!
From her POV, you’ve sent a message to her spouse saying SHE is toxic and controlling. That would have been incredibly hurtful and threatening to their relationship to read.
You’ve also said she can’t organise a do because the whole of November must be about you- have I read that correctly?? If so, I am genuinely very sorry you’ve had cancer but I have never known anyone make that sort of statement, and i have a lot of people with cancer in my life, including in my immediate household.
Was she invited to your do? And f so, surely it’s a simple “Sorry Maud, not sure if you’ve realised but that date clashes with my birthday party?” … and if she hasn’t been invited, how was she to know?
Or is it that you haven’t organised anything for your birthday yet but just expected everyone to remember that November is “all about you”, year after year?
I’m sure you don’t mean to but from an outside POV, you are also coming across as pretty controlling, and I do think your message to her didn’t land as humorously as you intended.
I think she has reacted very strongly, but I don’t think it’s as unwarranted as you do, and I do think you both have elements to apologise for.
This argument mostly punishes your brother, your nephew and your wider family. It comes across that you are not willing to own your part and try and put that right. You can’t control whether she does, but you ARE in control of whether you do.