So remember I'm a problem solver.....what does your husband say when you tell him how much it upsets you to have his mother do this?
Usually it's "She's her grandma. My grandma used to let me get away with lots of stuff."
It's not really a problem per se (and I'm sorry, I didn't mean to hijack the post or anything). On the one hand it's irritating (hence the vent), on the other, perhaps it's an opportunity to teach Peanut that at MIL's house we go by her rules and at home we go by our rules.
For something more minor, like getting up from the table before the meal is over... I'll let it go. If it's something bigger than that, well, we'll have to play it by ear. I'm sure if it was a major issue that H would be supportive.