Well-said, BradfordLass!
Kircubbin, so sorry this happened and hope it will blow over soon, as you predict. Please know that you are not alone in having this kind of "sensitive" DIL. Ive seen this before online and I have a friend who has a DIL like this - insists on spending every holiday, etc. w/ her relatives, but gets miffed if she finds out DS' relatives went ahead and celebrated w/o her/them. What do these kind of people expect? That DS' side will skip this/that holiday b/c she and DS aren't joining them? That they will freeze in place until she and DS choose to give them some time? I just don't get it, I really don't.
As for your phone conversation w/ DD, I take it you were arguing over whose "fault" it was. IMO, it was a picky, petty argument that should never have happened, but, unfortunately, we all get caught up in those sometimes, so I understand. True, she could have invited them, knowing they wouldn't come, but she didn't "have to." And true, it would have been better, in hindsight, if you hadn't mentioned it to DS, but you shouldn't have to watch your words, especially when talking to your own son. Granted, one usually doesn't tell someone about a party they weren't invited to, but DS and DIL must know the rest of DS' family do Xmas w/o them, so, IMO, it shouldn't have been such a big deal. IOWs, in my view, it was no one's "fault" but DIL's (for her immature, unreasonable reaction), and I'm very sorry you and DD ended up blaming each other.
Also, I understand about making plans for Xmas in August, especially where plane travel is involved. IDKY you were talking to DS about your Xmas plans so early since he's not involved, but again, I don't think it was such a major issue.
IMO, of the three young people - DS, DD, and DIL - DS has behaved in the most mature way, solving the problem w/ DIL. At some point, I hope he tells her that if she/they consistently choose not to go to certain events, she/they can't expect to continue being invited. But that's up to him.
At this point, though, I think you need to stay out of it period - as in, don't even make "suggestions." DD, DS, and DIL are all adults. Please let them solve this themselves.