Sam, knowing you like I do (which, I know is only over the internet, but still,) I am not surprised that you are feeling guilty about this. But honey, let me say that you are 100% NOT being selfish here.
Listen - Christmas is all about feelings. There's a big gamut of them, all the way from "hi, happy holidays" to dining together, opening gifts, and spending the night. Your MIL has chosen to pick THAT end of the spectrum and is basically holding you hostage for that by not accepting anything else. She can't actually say that you can't come into her house to see her, but she can refuse to fix a meal and instead of watching the kids open their presents, mail them to you.
This is her "strike" and she wants to do this to punish you for having your own Christmas.
Sam, go ahead and let her.
Perhaps next year she will ease up and not act this way. Ignore the implications here, have the kids write thank you notes the same day they open the presents, and give her the notes when you all walk in the door.
If you change your mind and let her treat you this way, then you will never get your Christmas morning to yourselves, because she will punish over and over. And once she knows this WORKS with you, I wouldn't be surprised if she did this kind of thing about other stuff.
Sam, she is being incredibly selfish to not be willing to have dinner later in the day and even worse, to refuse to watch children open presents. That part in and of itself is just terrible.
If I were you, I'd go the day after Christmas, stop at a nice restaurant in town before getting to her house, and enjoy your meal. Then no one is hungry. Bring one of the presents she gave the kids with you, and let the kids pick the other ones they want to bring to play with, and let them play with these at Grandma's. Ignore everything else. If the other family members don't come over, you can make the rounds to their house if you want, or don't. (Maybe you'd like to miss your SIL this time around? LOL!)
Please don't feel guilty. Your MIL needs to be put in her place. And next time, put your husband on the phone and let him be the one to tell her that if this is the way she wants to treat her grandkids, fine.
Now I understand where your SIL gets her drama from...
Hugs,
Dawn