this might sound like a stupid question - have you been honest with her? ie, when she makes plans like this ask her 'are you sure you can cope, remember how stressed you got the last time' and make no bones about the fact that her martyrdom is a pia?
having said this, the only time i have tried to honestly address my mum's martyrdom she burst into tears.
Tell them that you don't want to spend christmas day being bitched at so you're going to enjoy a lovely peaceful time with your family. You're an adult, you don't have to spend the day with two people who get pleasure in making you feel miserable.
oh dear. toxic parents. the only thing you can do is decide what you're able to change, do etc etc and that's so much easier said than done. or just put up with it and come on here for some moral support!
Oh, I recognise the passive-aggressive refusal of ANY help followed by the complaints and moaning and martyrdom... my mother is scarily similar.
The worst Christmas was when she stayed with my sister and it was daggers drawn in the kitchen with BIL. Two cooks sharing a kitchen and competing with each other over Christmas dinner is NOT a good thing! (I escaped the worst by staying at home and only going along for Boxing Day...)
YANBU. My mum does this too and she misrepresents what people have said. I always remember when i was at school and i had a French exchange student to stay. Every morning she would ask if the student wanted a cooked breakfast (fry up) and the student would sort of agree politely. She then went around moaning to everyone that the student EXPECTED a cooked breakfast every morning. As if a French person would expect a fry up for breakfast! She does things like that all the time. Generously offers things and then if you have the audacity to accept she starts to back out and moan to people that you are expecting so and so.
So, the first time you offer help for Christmas and she turns it down, say to her 'OK, just writing down that I offered help which you have said no to [and DO write it down] so that when you completely exhaust yourself and tell me I'm no help, I can show you the list of 23 times over the next eight weeks that I will have offered to help' [BIG laugh here] [change subject]