Hang on, before someone tells the OP to go NC and calls the MIL unspeakable names, maybe she has a good reason for kicking off.
I almost always have a little soft spot for us MILs on MNET as we get such a hammering. And usually it is from people who are not MILs themselves and have not got much idea what it is like to be one. it is not an easy relationship to get right.
More often than not MILS have to fit in with a DILs agenda. Perhaps this MIL feels marginalised and hurt. Perhaps she has been sidelined in the past. Who knows.
Without exception, all my friends who have sons say exactly the same thing. A relationship with your DD and their partner and children is NOT the same as with your DILS and your son. Everyone knows the old adage so I won't repeat it.
The MIL is getting two and a half days. On one of the half days she will be in the car. Yes, it is due to the kindness of her DILs' Mother, but it is not long is it ? Especially if she is usually alone and a long way from her DGC. The OP is staying much longer ( 7days) with her own DM.
I think the OP is dictating terms to this MIL and is inflexible and if I was her MIL I would know that I wasn't really welcome. The OP is telling her that they have plans that don't, and won't include her. Even though this woman has begged to be included, the OP is still refusing. Mmmmm Christmas. Couldn't her DH drop her off at her Mums and then go and get his Mum.
It is sad really, as I am sure that the OP thinks she is being really kind and generous to her MIL. She is also having to put up with the poor woman for a few days over New Year. From the tone of the the OPs post it all sounds a bit 'martyr'
Anyway, what ever happens now, Christmas is ruined for someone in this family. They all need to chill out and get some relationship counselling.
Thank God that I have daughters as well as sons.