Not my wife, but my sister-in-law who is mainland chinese.
She is going back to China (from Australia) on the 6th of February. Chinese new year is 8th of February. There is no apology no explanation, just she is going and my brother is going with her.
I am certain she did it intentionally. There are countless disrespectful behaviors and mind games from her that are just too many to mention. Even on the wedding day she intentionally introduced her aussie "foster parents" then her friends and finally my parents (the bridegroom / male side's parents) LAST as if its an afterthought.
She is just rude and disrespectful to say the least. I just want to rationalize it, to seperate her behavior as just her, not mainland chinese culture.
A lot of factors come into play when it comes to major holidays and visiting both sides of the family, especially for immigrants or anyone who has family far apart geographically either within the same country or internationally. Some examples I know of personally are to split holidays (Thanksgiving vs New Year's), or days within the same holiday (Christmas Eve vs Christmas Day), or even alternating years when it's not practical to travel to multiple distant locations for important holidays.
Most of my Chinese friends including ones from mainland China go by a mix of closeness (personal relationship closeness) and seniority. First it's each others' parents (doesn't seem to matter whose), as well as grandparents since they tend to live with the parents, then it's other relatives who are their parents' or grandparents' generation who they actually have a personal relationship with. Usually there is an entire extended family get together on each side of the family so they don't actually have to visit everyone personally in order to pay their respects to everyone. But these are situations where both sides of the family are nearby.
You should definitely keep your judgment if it's solely based on her behavior as personal to her rather than generalize. It's also just a part of the overall personal relationship between your sister-in-law and your brother, and the rest of your family.