I would start to let the DCs know gently that the outlook for their DGM is not good, look at Winston's Wish website for some help on language and helping to prepare them.
A DGAunt died, after a long illness and well well into old age (aged 97), on Christmas morning. She was a nun, and both Christmas and Holy Mary meant a LOT to her, and she slipped away in the 15 minutes that another nun sitting with her had gone to the loo and to make a fresh cup of tea (there was no indication that it was imminent at that stage, and wider family had not been called, but were called for a couple of other "potentially this is it" moments months earlier). It was a blessing for her, and actually, meant a lot to family that it happened that day. We were all down 2 days later for the funeral and it was lovely.
If MIL is very unwell, and you are visiting, give her "permission" to go when it suits her. IF it happens on the wrong day, it will be sad but it doesn't mean that is the end of Christmas - yes it changes it this year, and next year too. But it doesn't mean that you will never again celebrate or enjoy it.
One thing we always do on Christmas Eve is light a candle, to show any weary travelers that there is "room in our Inn". And as part of that, we spend a little time contemplating the good and bad things that have happened in the year just finishing, and also every year, remember family members who have died (including all of my DGPs, that DD knew 3 of them well, and this year will include DFIL and DH's Aunt as well). For us, it marks a lovely time of quiet rememberance at the start of the real celebrations, but doesn't detract from the celebrations either.
There was very little opportunity to plan ahead much for DFIL either, there were 5 weeks from cutting the grass on Thursday night, going into hospital on the following day, and him passing away. But he did give us some time to wait with him quietly, with 3 days in a peaceful coma, where absolutely no planning was possible beyond which family members slept in hospice that night or who was going to bring back dinner. That is very hard to do, and both DH and I are the planning and organizing types (and the apple didn't fall far from the tree in DH's case).
Just have plans in your mind about what steps need to be taken, like does MIL have a favoured funeral director, cremation or burial, favourite readings/music. Who needs to be contacted (can you set up a circle of - DH tells 1 person, who then has a list to contact and let them know etc to share it out, have you contact numbers for all important people including mobiles in case of Christmas travels)? What to tell the DCs when it happens, as opposed to preparing them for the idea. And having some idea of clothes that you will all need - do you need to get some formal things for the DCs (whether that's nice jeans and plain top, through to formal suit and dress - depending on what the family is normally like and what you think they may expect, particularly your own DH's expectations), do you have suitable things yourself and are they clean/ready to wear, will DH need a new suit or shirt, that sort of thing. (List for sales to pick things up may be handy). And having options in your head for catering when things happen - and you just can't face making dinner but suddenly there are an extra 12 siblings, cousins, aunt and uncles descending and all the neighbours calling, or you may have to decamp to MILs for thigns....how will it work in your particular family?
You don't have to write anything down, but a list can help you go onto autopilot. I have what I literally call my autopilot list from when my DGPs were ill, as I live in their city but most of my family don't so needed to prepare for invasions of visitors, probably a number needing beds, probably a number arriving off planes needing to refresh themselves before church, and feeding ourselves and visitors around it all. And by thinking it through beforehand, I had time to build in what I needed to tell DD, having easy to make meals listed (including a shopping list of ingredients), and step by step instructions for myself. It also meant that when it did happen (3 times), I just took it out and followed mindlessly, so I could think about my DGPs rather than "what's next".
But before all that happens, visit MIL as much as is feasible - from her own strength and awareness perspectives, and your ability as a family to get there. You probably won't care what day it is when you get that call - but you will feel better if you've had a chance to visit and spend time with her.