@maryanne3 you wrote:
Reading between what crumbs have been dropped on this, I think there is at least one (single) sibling who gets quite emotional about them all being together and having a childhood Xmas with stockings at the end of their beds (they are all in their 30s) etc. So that may be partially it. But I do resent (quietly) that my family Xmas is always cancelled/moved because one sibling insists every year.
I don't think you're being unreasonable to be disappointed. However, it doesn't matter what you expect. DiL will spend her Christmases in the way she sees fit - she's made it pretty clear this is non-negotiable - but this doesn't mean the same consideration doesn't apply to you.
If you see your son without her some Christmases, you don't really need her presence as well. You can't make her come. You can, however, celebrate your Christmas in your way at your home with those who do accept your invitation. You can change the mindset that this occasion will either be a moveable feast or repeated - at considerable cost and work to you - on a separate occasion to indulge this. If they choose to visit on a separate occasion they could find themselves doing something else - a chili or curry night on the day you exchange gifts, for example, or a meal out at the local. If they want something 'Christmassy; a slice of fruit cake will cut it. They are no more entitled to create grand expectations of you, any more than you are of his wife.
But on the latter point you are not being at all unreasonable. If anything, it might be an indication that you've bent over backwards a little too much. Unmet expectations can cut two ways.
You can't change others' behaviour, and after 16 years it's unrealistic to think you can make her change. You can, however, change your own, and set your expectation levels accordingly. I hope at some point you can do this, and find some peace in your decision. Wishing you a happier festive season for 2022.