The other thing which jumps out is the ''i want what she's having'' referring to DILs mum.
This is poison. Seriously. A bad mind set.
The years where your son has stayed with you on xmas day She (the other mother) hasn't had it all. She hasn't had ''everyone'' there. The years he has gone to her's, maybe she has? Maybe all her dreams have come true?! But it's not every year of these 16 looong years thing, is it? And some of her kids live abroad now ... . Anyway - don't even get started into this counting and comparing business. Madness lies there.
When 2 families collide there are a thousand different nuances between relationships which open up. When there are siblings getting married (like your other DCS) 3, 4, 5, 6 families are colliding and intertwining. Your other DCs will have thier own in laws to negotiate with.
Some stuff is hidden under the surface. Here's a quick example of one of mine. My eldest is engaged. They have bought a place. He's a lovely bloke, but on the surface he seems a bit shackled to his parents. It would be easy for me to think ''i'm sick of his parent's views being held sacred. I'm sick of his parents being the ones put first so often''. BUT - he's an only child. Now I know what it's like to be an only child because i am one. I was the apple of my parents eye and there was no siblings to be interested in - just me. I've got 4 DCs, but for them there will be no other weddings, or DC moving outs, or xmases with other DCs for them - it's just him. It was like that with my parents and me. I'm not going to push against that with them. I know your DIL is not an only, and that isn't the point. I'm saying you bend with the wind. There just isn't fair or not fair in some situations. Don't let it drive you mad.
If he did have siblings who lived abroad and who all came together on xmas day at his parents and he wanted to spend every xmas day there i'd get it. As it is sometimes DD1 wants to come here to have dinner with her family and sometimes wants to go there and have xmas dinner with him. No probs. I have other kids! And so do you.
My DD2's xmas is also complicated. Her MIL, like you, has all sons. Some of them are flaky and turn up/don't turn up on the day with no notice. For the last 5 years DD2 has stayed with her MIL, at her request, for xmas dinner. She likes having a girl (my DD) around. DD2 will then come to ours asap for the afternoon to be with her siblings. FIne. (this year she's coming to us for the day. and that's lovely, but not her fiance, he's staying with his mum). I've told them all to do what they wish - the door is open all day. Just let me know by mid Nov for food catering.
If DH and i ever find ourselves alone for xmas we'll bugger off to a lovely hotel and have someone else do the cooking 
(although with DD4 still being at primary age that'll be few year yet)