Jokes and fairy tales
I never actually took a course on the study of fairy tales. But from what I understand, there are many psychological reflections built into them. Among them is the wicked step mother. I recall seeing that all the negatives one would associate with a parental figure are projected on the step-mother because it is too much for one to admit it in a biological parent. Hansel and Gretel would seem to be an exception, unless the mother there was supposed to be a stepmother, as well.
In modern pop culture, fairytales are often supplanted by stereotypical jokes. And there are a slew of MIL ones. But I think that the projection dynamics at work are actually the other way. Just as the "bad parent" has to be made into a stepmother, the "bad child" has to be made into a daughter-in-law. That way, one's biological child can still be idealized as perfect, brought down only by an influence that is alien to the biological family.