I have two kids. My daughter, the younger of the two, learned all the right lessons, and she has been a model citizen all her life. I could not be more proud of her. Her brother is 16 months older, and for reasons I'm still not sure about, has been nothing but a pain since he was a teenager. Problem after problem, failure after failure, trouble after trouble, arrest after arrest, and none of it, of course, was *ever* his fault. (Yeah, right.) He's now in his 40s, and is nearly homeless. He stopped coming to me several years ago when I told him I was not a bank, and that I was tired of him only asking me for things - usually money. When I told him there would be no more money, he stopped coming around. I guess he's trying to teach me a lesson. Not sure. It took me a long time, but I learned there is no shame in telling an adult No. He knows I love him, but it is not my responsibility to make his life work. He's on his own, as far as I'm concerned. Maybe I'm trying to teach HIM the lesson. But it is what it is.
My advice to you is to stand your ground. If you want a family gathering for the holiday, tell the collective family that your gathering will be on a certain day and time. Everyone is invited to attend, and if they don't show up, well, that's their loss. Tell them your grandchild's gifts will be waiting at your house. Don't send anything to your son or DIL, regardless. Make them come and ask for it.
If you want to play Devil's Advocate, when they do come over, take your son aside privately, and ask him how long this charade of an abusive marriage is going to last. That should be grounds for a spirited discussion. On the other hand, he may be looking for a way out, and might just need someone in his court. It's a thought.
Good luck. I know it's painful.
Dave