I have two--a stepmother-in-law and the regular sort.
The regular sort, DH's mother, is very sweet and as polite and thoughtful as they come. She sends lots of handwritten letters and when she visits she is helpful and insists on cooking us dinner (with our eggs). And she's a good cook.
The SMIL was banned from my house within a year of my wedding, for 1. attempting to foist her teenage daughter off on us to raise 2. telling us that our religion was evil and horrible and attempting to shove her personal made-up religion down our throats. Yeah, we're evil horrible people--now please raise my 13-year-old daughter.
I'm convinced she is mentally ill, for a lot of other reasons, but after a while, the compassion for untreated mental illness, it really runs out.
What is it with the egg thing? It's the same with meat and milk, I swear. I know more than a few people who figure that meat comes from the grocery store, and if you offer them anything home-raised or hunted, they freak out.