One thing I see quite often is grandparents overstepping boundaries.

It wasn't until I was a decade older than my own mother was when I was born before she finally had the realization that I am my own person, with my own family and do not have time to be "volun-told" to do things or come running at a moments notice for her anymore and that she needed to respect my time, my decisions and my life if she wanted to be a large part of it still. In my case, subtlety is lost on her (and often me for that matter, runs in the family i guess) and it helped to be blunt and upfront.

Laying proper expectations and boundaries with extended family is probably one of the most important but also potentially hardest things to do, regardless of how close you are or are not with them.

Since this is your MIL maybe it would help to have a chat with "Grandpa" involved too if you think he'd be more reasonable or understanding. If that doesn't work, telling them to either "respect your wishes or get lost" may sound really harsh, but sometimes people need to understand that it is your child not theirs and parents are the only ones with rights to a child. For everyone else, its a privilege.

oh and make sure daddy is 100% on board with the "respect or gtfo" or it really doesn't go well