Sure, and like I said, I'm veering off the topic of the OP a little and talking in general. Levee Woman suggested that children shouldn't have to plan their holidays around spending equal time with both parents and no, of course they shouldn't, but I think, all things being equal, it would be a kindness to try to share out time as equally as possible or, if you do spend a significant amount more time with one set of parents, to not let the other parent know that. I can understand hurt feelings, is all.
Of course, how the hurt feelings are expressed is another matter entirely and when the hurt party is churlish or childish, yes, that's very rude.
I think it's the "all things being equal" part that's the issue at the moment. In the past, things were relatively equal, and they divided their time between the two sides of the family equally.
At the moment, things are not equal. They can afford to visit the OP's family. They cannot afford to visit MIL's family. If they don't go to visit the OP's family, it won't make any difference to visiting the inlaws - they still can't afford to go. This family visit doesn't take anything away from MIL - the only thing refusing to go would do is give MIL the petty satisfaction of "If I can't have something nice, nobody else can either".
If the MIL offered to pay for the family to visit her now, when they have time but no money, she could get her visit. It appears that she doesn't want to do that, and her visiting them at a later time at the OP's expense isn't good enough either.
I look at this from the perspective of someone who has very different visiting situations between my family and my inlaws. My inlaws are much closer, and easier and cheaper to visit. My family is significantly farther away, and a *lot* more expensive. So we see my family less often (only about every 1.5 years) but for a longer period of time, and my husband usually manages to stop in to his parents a few times a year on business trips (something that never happens to me). My husband can also talk to his family much more easily (same time zone vs 9 hours difference).
For my parents - we lived a similar distance from both sets of grandparents (they lived close to each other, but far from us). We visited them with the same frequency (every two years). But my mom's parents could travel out to visit us, while my Dad's parents' couldn't, so we saw the former set twice as often than the latter set. Denying us those visits with the grandparents to keep things fair would have been petty, and a net loss to us.