Well, you could, but as I hinted in my first post, I told all family to sod off, many years ago.
DSis has had a birthday party at ours, when we weren't here, though!
Why all the angst at me? Is offering A N Other perspective so very bad?
As far as you all know OP hasn't even read my horrid suggestions yet. Or she might have and decided to tackle her DH, when they are both less fraught, and sorted it all out, amicably.
There really are only 2 choices: a) sort it out like adults or b) let it cause an argument.
The outcome doesn't really matter, what does matter is that OH and her DH come to an agreement. At the moment they have both let their ire be known, MN has told OP he is a shite of the first order and she should put a stop to his thinking he is an equal partner.
They are BOTH wrong at the moment.