It is that way for Christmas too, where BIL does all the cooking and everyone waits for him to finish cooking and serve. It seems odd to me, since family holiday meals should be about the whole family. I have known other people who seem to have either a spoken or unspoken rule that whoever hosts does the cooking. I suppose what frustrated me most was that instead of being allowed to introduce them to foods that go with Thanksgiving, since they went to the effort of having Thanksgiving, it was more or less a turkey Sunday roast on a Thursday evening.
I can understand your MIL wanting someone else to take a turn so she can relax for once, but overall, I love the busy kitchen feel of everyone pitching in, as a family.