I think that it’s really sad that you have inadvertently prioritised your DM over your DCs and your own needs for many years.
Your own DC are asking you to prioritise them for once.
Just do that.
Craft a clear, direct message and put it in the group chat - with your DM on it so that she can see who steps up (or not).
Do not take responsibility for sorting this - pass it back to the family.
Don’t feel any need to JADE (justify, apologise, defend or explain) your choice this year to anyone. Your siblings haven’t felt the need to.
“Hi All, I am spending the Christmas season with my immediate family this year.
I will leave it with you all to make appropriate arrangements for Mum.”
Close down any Qs quickly by repeating a version of your message - cut and paste if needed, don’t get drawn on any further detail and ignore if the Qs are repeated.
“Why?” Because I am spending the Christmas season….
“But what about…” I will leave it with you all to…
“But I can’t do it..” - ignore
Sounds like you have given more than enough to your DM and shouldered the responsibilities for your siblings with zero gratitude (+ simmering hostility) at a great cost to your own DC.