You have quite a big lead up time to things, don't you?
Being told that people may not come two weeks ahead is good going in our family, for example. But clearly, if you organise on a 4-months-ahead schedule, that two weeks notice of cancellation is going to look rude and flakey by comparison.
I have a friend a bit like you. She would book my husband and my son for her son's birthday absolutely months in advance. She wanted my dh to act in the performances she arranged for her ds' birthday parties, and she felt she had to invite ds along too, since he was the same age as her son and technically they were friends.
Unfortunately, one of son's best friends' birthday fell around about the same time. Best friend of ds' mother was less organised. About a fortnight before the event, she would invite ds to his best friend's birthday - inevitably on the same day as Organised Woman's Son's birthday party.
Ds missed his best friend's party for four years because none of us had the nerve to cancel on Organised Woman. And of course, since she booked dh and ds several months in advance, we didn't feel able to say: "Well, here's the thing: although it hasn't been arranged yet, we think there may be something else ds (and dh actually) would rather go to on that date."
Four months ahead of Christmas is still summer, really. I think you may have to accept that some people are less good at organising than you.
Terrible shame about the food but can't you freeze whatever wasn't eaten on Christmas Eve?
By the way, there is the outside chance that your sister is cross with you because your Christmas Eve do may have looked like a strange "spoiler" of her Christmas.
Still, on the bright side, aren't your rellies lucky that you all want to look after them and see them at Christmas?