C.C.
C.,
I feel your pain. A little background info about myself. I've been married for 17 years and have two mothers-in-law due to my husbands parents being divorced and remarried. Holidays are especially hard dividing time with everyone equally. (We've never lived close so it's always an overnight visit)
If you've only been married for 18 months it's possible that your MIL is still adjusting to her son being married. She needs to realize that you are a family unit. You did a wonderful job offering Thanksgiving versus Christmas and should stick to her original request. But, if the decision has been taken out of your hands by your husband you need to discuss with him about presenting a united front to his mom.
My advice to you is to keep your mouth shut and allow you husband to be the bad guy in these situations. You will be dealing with your MIL for many years to come and don't need to exchange bad blood with her. She will forgive her son but not the wife of HER son (if you know what I mean) :)
Give her time to adjust to your new status but don't rub it in her face. One day we will all understand how hard it is to turn out "babies" loose to another. Until then cut her a little slack, this year, and make sure your husband sticks up to her in the future.
Coming to an agreeable solution for both sides is key.
Good luck & Merry Christmas! Let us know how things turn out. :)