Hi KatyK no you are not the worst mother ever but we all feel like that sometimes.
I suspect sil's family had already made the invitation (this has happened with my DD and her inlaws too) and whatever she says abut them to you, it is just a 'moan' on her part. My DD also moans about her inlaws and yet you would never know it if you saw her with them.
For the past few years, until DD and SIL bought their house this year, they spent every, yes every, Xmas with his family in Wales, and so I never saw them at Xmas, just made a pre Xmas meal for us all before they left. Last year was the worst - when they took 3 month old DGS with them.
This year is the first for 6 years that we will be together (minus ex husband) but I am realistic to know that this won't be the same every year and in fact will probably encourage them to have Xmas Day as a family on their own if they want in their own home, in the future. If they invite me, that will be great, if not we will all see each other on a date as near as poss.
So i guess what I am saying here is, bite the proverbial bullet and ask your DD which day you CAN all be together instead. At least you have your DH with you on Xmas day, which many of us haven't, so that should be some consolation, no?
btw I always feel I am the one who makes concessions to both DDs! You should read some of the posts on Mumsnet to see what many DD and DiLs feel about us. Most of us seem to be the devil in disguise. Your hair would curl <hmmm>
we really do need a middle of the night moderator