Firstly, you’re not responsible for the rest of your family, what they do or do not do. And, frankly, THEY don’t OWE you any thanks for what you do for your grandmother. If her being your grandmother, and appreciating what you do isn’t enough, then I wouldn’t do it.
Secondly, it sounds as if you go to see her regularly, and know her and she knows you quite well. Why don’t you have an honest conversation with her about how you feel? Just something along the lines of, you know I’ve not been too well this year, and I’m stressing about the rush on Christmas Day. Why don’t we come over and have a few hours on Christmas Eve with you, you could do present opening with her, and take her a meal for her to eat on Christmas Day. As she sees other family members on the day, she won’t be lonely, and spreading Christmas over 2 days would be nice for her and for your children, and reduce the stress for yourself.
It can be hard to accept, but your parents are not going to change their attitude towards your grandmother. None of us know why they have the attitude they do, but that’s irrelevant tbh. You do you, and let worry about themselves. One day, you might be the daughter who doesn’t want to bother with their parents on Xmas Day.