You can politely say that it is important to you to also spend time with your family on Christmas and remind her that you didn't get to see your family last year. Adults can be like children too. If they get their way because of bad behavior than the behavior will continue.
Personally I probably would have said something a lot more harsh and left if she didn't stop, but that's probably not the best thing to do. I think your husband needs to take charge. It's his responsibility to manage his family thus his job to explain to his mom that now that he has his own family he is going to have to split time between both families.
This, this, this. I wouldn't have even gone to your IL's house this year since they got you guys both days last year, but that's just me.
Your husband needs to stop allowing his mother to have such ridiculous behavior. He needs to say, 'Mom, I'm sorry you feel that way, but I have TWO families now (almost THREE!), and I have to do what is best for all of us. We are going to be with wife's family this year b/c we saw you guys last year. If you don't like that decision, then we don't need to come at all."
DH has had to do this before, and although the drama with the ILs still continues, it is much better than it was. Our families have both been told that we will do what is needed for OUR family first, and if we can find time to see them, great. If they can't meet up when we can, then we will see them another weekend/holiday.
It isn't about just them anymore, it is about what is best for YOUR family. Your MIL had small children once, she knows- she just doesn't want to accept that her "baby boy" has grown up.