M.D.
You are not crazy, and if you stand your ground, she will get over it. We do exactly what you plan to do this year. We visit 1 family the weekend before Christmas, and 1 family the weekend after Christmas, and we spend Christmas eve and Christmas day at our own home. The first year, no one was happy with us. The second year, there was a little pouting but it wasn't so bad. Nonetheless, we stuck with the plan and developed own own traditions on those days. Now it's 10+ years later and everyone just asks - what weekend are we celebrating Christmas this year?
So my advice - simply continue to stand your ground pleasantly, refusing to engage in arguments. If your mom tries to pick a fight with you about it, simply say "I know you are disappointed, but this is our plan from now on. I hope we'll see you at Mike's home the weekend before Christmas."
Enjoy your day! It is really lovely to make your own traditions and relax at home on Christmas.
ETA: After reading responses below, I'll make one addition. Every year we tell all the parents our plan and make it clear that if they want to see us on Christmas day, they are welcome to come to our house and join in with what we are doing. They have never taken us up on it. But it gets around the "you are depriving us of our grandkids on Christmas" arguments. We aren't! If they really want to see their grandkids on Christmas, they are more than welcome to come to us. It turns out that roads go both directions!