I guess if I were sick on Christmas day, I might want my family to be around to wait on me a little with chicken soup and pain relievers. Maybe I'd really want the whole thing to be rescheduled for the next day even if my recovery and strength were questionable, just so Christmas wouldn't be totally ruined and all that food gone to waste. I might figure on toughing it out, and make my husband do the decorating that I hadn't gotten to because I was so run down.
It's also possible that something else was going on - but I'm not sure what you suspect in particular. That they had other guests? That they wanted to do their gifts in private? That they just hadn't gotten their act together to do all the cooking and shopping, and they were stuck on Christmas Day with no stores open? Do you suspect that there was a hangover or a bi-polar type episode as suggested by other posters?
I'm sure it's possible, but you haven't really said what the "clues" were other than the fact that she seemed totally fine the next day. It's also possible, from what little you have said here, that she was using every last bit of strength on 12/26 to make it happen so as not to disappoint you all.
If there is more to these people than what you have said here, there may well be history that none of us have. Maybe you picked up on some superficialities or duplicitous attitudes on their part that I just can read into your question. You would obviously be much better at that than I would.
I think you have 2 choices: ignore it and move on, or try one more time under the "fool me once, shame on you; fool me twice, shame on me" theory. Either way, you will have either zero chance or just one more chance of a repeat, and then you never have holidays together again.