Mil is probably having all kinds of emotions..
Seeing how you two don’t get on already I’m not surprised.
Her blackmail is not healthy. Not justified. But just shows she is feeling intense about stuff.
I wouldn’t give in to follow her orders.. but I would use this occasion to forge healthy relationship with her..
I would have my DH speak to her calmly, let her express her feelings. Acknowledge it. Express yours. And let him make a final decision based on that.
I really think on this topic you should interfere as this is his Uncle, from his mums side.. and his mum being there matters to him so he might want to make that compromise and that’s his choice.
But I would advise him to not give in until his mum turns blackmail into a healthy normal conversation.
So :
DH: mum, I would like to make my own choices about who I invite. Inheritance and your attendance shouldn’t be used against me and I cannot honor your wishes while you disrespect me like that.
I would like for us to be on good terms during my wedding and beginning of marriage. I don’t want this to drive us apart.
When you are ready to have a respectful discussion while taking my decisions into consideration then let me know.
2 days later.
Mil: beep beep... I wanna talk.
DH: ok mum, just remember I’m an adult and you don’t need to use blackmail as a naughty step.
Mil: states her feeling
DH: I understand. You are right it would’ve been nice if your brother could be with us. But this is how I’m feeling ........
Bla bla
DH: mum I truly hope you can respect my decision
(Wife should respect it too if he ended up wanting to compromise).
DH: see mum, things work out better when we hear each other out without threats.
THE END.
I would honestly honestly try to contain the situation this time because it’s sensitive and raw and not worth starting on a bad note.
And there is potential to gain her on your side, while you are both respected if she can build a relationship with her son and talk things out while feeling heard