DH and I have been married 12 years (as of August. Hooray) and she and I have had an "interesting" relationship. This year, especially the past few months she has been ... challenging to deal with. (no, nothing medical that we can tell, just getting crankier.)
Yes, we tend to see her quite a bit more than my mom. Was going over every week or every other week when we were first married and then dropped to once a month plus things like birthday parties, family get-togethers (reunion, Superbowl, etc). When we were going over once a week, she would "ask" DH to do lawn stuff or other honey-do list things and demand he come over at a drop of a hat. (That has been curtailed few years ago.) DH found that she is easier to deal with when we only go over once a month.
Even if we spend the majority of holidays with her, she gets bent out of shape if we go visit my mom for a long weekend. Even if it for my mom's birthday.
Normally, we just have told her that we made plans for the holiday about a month before, but for some reason, I am thinking this year she is going to be .... irritable/prickly/touchy/petulant (pick a favorite) about it. We will be seeing her this weekend at a family reunion (her side of the family). Perhaps we will broach the subject when we are getting ready to leave.