It was more important for her to spend Christmas with her parents than support her fiancé because it's the first one after getting engaged? What the hell kind of logic is that?

I've been in the same room as a mother giving advice to her soon-to-be co-habitating daughter to push for her man to do whatever chores she doesn't like for the next week or two, because once he does them once, the pattern is established that it's now his responsibility forever. This is probably something similar. She doesn't want to spend time with his family, so she wants to set the pattern immediately.