I would also like to add that indirect communication seems to play a big part in the problem here. Some families like to use the grapevine for some information--like pregnancy announcements, for example--and that can be fine. But, when someone is trying to plan a get-together, having intermediaries between the attendees and the host/home provider (within a small family group) can in some cases just lead to confusion and miscommunication.
Everyone in this story is an adult and should be able to communicate directly about their plans, instead of saying, "Auntie, tell your daughter to tell her sister that we might or might not be coming to spend Thanksgiving with her this week..." I would, I think, be further insulted that someone who was perfectly willing to partake of my food and home could not tell me directly they wouldn't be there, but had to pass the message through others. (Or maybe it was the OP's sister who passed a message through her mother? I'm confused about that part. But either way it seems like no one who was part of the original plan communicated directly with the OP to tell her it had changed.)
Going forward, if you find yourself wanting to host people again, I think I wouldn't let someone else coordinate it, but rather would directly communicate with all adult participants, and follow up/hold them accountable if they don't do what was agreed to. I don't think the OP did anything wrong on this point since it's likely the usual family method of communication, but it might help improve things in the future. My own family is frustratingly non-communicative and indirect as well.
