Is you get to make the rules
. It's also one of the worst things, as sometimes it's TOUGH, really tough.
I would look for ways to "wean" your child from her grandmother. Not deny them together time, not punish, but to limit it in ways that are acceptable.
I am both a parent and a grandparent. Sometimes, as a "grand" it's fun to be a co-conspiritor with the "grands". We have...perhaps once a year, snuck out for ice cream, or a slupee or something my daughters may not approve of! I am guilty, guilty, guilty!
Having said that, with the exception of these few moments, I always clear everything with my daughters before I take or do anything with a Grand...can they see THIS movie, is it okay to have lunch, can they have ONE glass of soda while visiting, ect ect.
Perhaps you can start adding some "other" after school activities that will allow your MIL to be separated from your daughter graciously. After school program, scouts, playdates...you get the idea. I might not be seen so much as punishment for Grandma as enrichment for your daughter.
Best of luck.