Argh. We have a similar problem.
Our families live in two opposite directions and so we usually spend Christmas driving up and down the country. Like PPs have said it is disruptive for our DC who just want to open their presents and play - not open presents, pack them away and sit in the car for two hours.
Last year was a revelation - we just stayed at home and the DC loved it. We have a bigger house now and so can accommodate my parents and my MIL (she is a widow).
However, my MIL point blank refuses to come if my parents are here. Quite frankly I am insulted.
My parents are not strangers to her. Whenever MIL visits they come over to see her especially- they exchange messages etc.
MIL just has the oink that we are not toeing the line. To her, Christmas is a certain way and that is it. She doesn't care that it is a three hour car journey for us (she doesn't drive).
If we put our foot down (my parents have already agreed to come - they usually go to my sister's house) then MIL will spend the day alone. My SIL has her own family traditions which is just seeing her own small family and only seeing us/ MIL on Boxing Day - every year we have to drive up to MiL at some point, she might get the train to ours but this year it doesn't work for various reasons.
So, basically, MIL has said she won't come for Christmas Day but expects DH to drive up to collect her on Boxing Day (5hr + round trip).
I have said if that's the case she can wait until the 27th.
I spend virtually every Christmas with her. I never see my parents, I usually try to see my family on Christmas Eve or in the days between Christmas and new year. This is the first year in our new house that we can entertain, our old house was too small.
The only reason I can see why she won't come is that we will have to give the guest room with en suite to my parents and she will have to have the single spare room and use the family bathroom. For one day. Surely you would get over yourself and suck it up for one day?