Well, based on how they are presently acting toward you, I'd guess she has told them you didn't want to go their house on Sundays.....and maybe Christmas, too. That's the problem, the in-laws get their feelings hurt. Even if they aren't hurt, they will side with their daughter b/c she's their child.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!