Mom wants me to choose between grandparents and dad...

my parents have been divorced for about 10 years now and while the agreement says that I should go to his house for christmas every other year on christmas eve & spend the night, we’ve just always gone over on christmas morning. this year, my dad asked if my sister and i (shes 20, im 17) would come over at 10am christmas morning, and I said it was fine because it’s really not fair to him since he doesn’t get us when he’s supposed to anyways. so I told my mom this last night and she send me this:

(the party she’s talking about is my dads annual christmas party that has well over 30 people at it, and is a mix between christmas and my dads birthday that is in late december, so it’s not like it’s a substitute for a christmas morning with 5 people at my dads)

she makes it seem like i’m PURPOSELY deciding not to spend more time with my grandparents when that’s the furthest thing from the truth.

I want to talk to her about this and tell her it’s not really fair, but everytime I want to bring something up about my dad, she acts like i’m the devil for even mentioning his name and calls me “selfish” and tells me that I have “no reason to stick up for a cheater” (which is absolutely uncalled for in my opinion, while yes it was shitty that he cheated, he didn’t cheat on me and has always tried to be in my life since they broke up)

how can I talk to her without blowing up her extremely short fuse when it comes to my dad?