Buy yourself a t-shirt that says "My Mother-in-law so uptight I can shove coal up her ass and she'll crap out a diamond".

Your husband took your side. That’s a good sign.

I tried to be good, but I take after my Grandpa! - So I'm excellent. Would have saved you a bunch of grief with this pair of overly-uptight individuals.

I don't see how is it funny, especially since older people seem to lack this you suck type of sense of humor. Also, the fact that your MIL automatically thought it's about her and not about your parents shows something about your relations. I would say dump the shirt and get a I love my grandparents shirt instead to show your apology

YDI. Grandpa could have bought that shirt himself as a joke, but you shouldn't have. If you just thought the joke was funny, you probably should have gone for the "... take after my mom" version.