For the past several years we've spent NYE with my sister and her family and each year we alternate whose house we go to. Last year we didn't get to spend NYE together as my DD had covid so my sister etc went to her friend's house instead. This year we agreed (about a month ago) that we would go to theirs on christmas day and they would come to us on NYE. My sister has just sent me a message saying that her friend and her son are very keen to join us for NYE and would we mind going to her house instead so the friend can come. She offered to come to us on Christmas day instead. I know it sounds childish but I feel a bit annoyed because she clearly invited her friend before even talking to us about it. She's already done this for another event (with the same friend) where we had booked to go to a Christmas show with the all grandparents and she invited this friend without asking anyone else. I don't know why we should rearrange our plans just to accommodate her friend - I was looking forward to not having to cook on Christmas day! I know it's not really a big deal and we'll end up agreeing to it because I don't want to be awkward but I feel a bit put out about it!