What is it with grandparents who want their grandchildren to stay overnight and so they can play mummy again! Mine love coming here but it's such hard work. I almost always agree to have them but have never asked for it. Again, I never ask to look after them but don't refuse when asked unless I have something else planned.
It does seems like both of them have been listening to the other DIL so there must have been more disagreements there than admitted to. However, I admit asking you to change the date of Christmas is ridiculous but, if your son is happy to always go to her parents, then that's the way it will be. Also, visiting at the same time as the other gran is asking for trouble too, too much scope for competition. It does seem like DILs always gets the blame when your son should have been brought up to stand up for his own opinions too, if they differ from his wife's.