Basically:

MIL doesn’t get on with her brother

Is angry he’s invited, but it’s our day

She’s repeatedly asked why he’s invited and tried to dictate seating arrangements

Fiancé told her not to make a scene at the wedding

She’s furious that he has even suggested this (despite the fact that she’s repeatedly initiated arguments with him about it - thus showing she’s capable of making a scene)

She’s threatened not to come to the wedding a few times over the accusation of her causing a scene. Yesterday she’s said there’s no way she’ll be coming due to being self conscious over how she acts/talks. This also means fiancé’s sister won’t be coming, as she 1) lives with MIL in the middle of nowhere and 2) always takes her side.

It feels like she wants an apology and to win the argument, which she won’t get/do. She’s caused problems with every other friend/ relative she had in her life in similar ways but doesn’t accept this as her fault / why fiancé would be concerned about her at the wedding.

I want them to be there because 1) if they don’t come, I think this will end their relationship with my fiancé and therefore me and my parents too (who also liked them).

2) I like them, and thought I finally had in laws who I would love and cherish, and wanted them to be part of my day and future.

3) it will tarnish the memory of the day for myself and my partner, especially as they’ve been so involved in planning and decisions

4) I believe she’ll genuinely regret not being at the biggest day of her son’s life.

What the hell can I do? My fiancé is “done” with her (as she’s caused so many issues like this over the years).