Laguna, I suspect Sparkle is trying to see if DMIL is being unfair in expecting 1 thing from her DS, but not having done the same when he was small, or if DMIL is in fact following on from exactly what was done in the past (that all important word - "tradition"!). There is nothing at all wrong with spending the day with your family, if that is what everyone wants and everyone is happy about it (and preferably there's some sharing of the burden between everyone too). But Cadbury would like one Christmas where it is just a relaxed day with her nuclear family - a different kind of Christmas day - rather than the one which she is being forced into by her DH. And that's where the discussion should happen, where both DH and DW can talk about it openly and express their wants, before reaching a compromise that each can be happy with.
Either way, I think you would be perfectly entitled to at least discuss it with your DH, OP. Maybe try and tease out what HE would like to happen, in detail.
What does he like about his DPs coming.
Is there anything he doesn't like.
Definitely remind him if he didn't step up as promised last year and make it clear that you want things to change.
You will not be working to anyone else's timetable but what suits the immediate family, particularly the 2 small DSs.
Can you put boundaries on it in some way? Having a gathering before or after 25th? Or limiting what they expect to get of the day - asking that they not turn up until a certain time to allow the DSs to open their Santa presents and enjoy playing with them for a while (and you getting to enjoy it too), being clear on when you expect them to leave to allow you to relax together, or that you will be playing family games rather than watching the Queen's speech (or whatever boundaries you need to make it the day you want).
Also, is it up to you to cater for everyone? Do they bring anything? Do they help? Do they make demands on the menu etc?
Definitely get DH into a conversation on what he expects to happen, and how HE will get involved, and also how THEY will get involved. Reminding him (again, if necessary!!) about not stepping up last year as promised.
Perhaps he doesn't realise exactly how much work is involved in "Christmas", as a whole, and in "Christmas Day/Dinner" specifically.
So lay it out, and with a list of jobs in front of you, agree who will do what AND WRITE IT DOWN! So he can be reminded that he agreed to to certain things, and you agreed to do others - but you can call him to account if he doesn't do his part, (perhaps even to the point of cancelling the day if its not organized?).
Come and join us on the Christmas Board, as there are lots of people who have had similar issues and there is lots of advice over there (all year round
but we do try not to shout it from the rooftops until later in the autumn).