MIL feels like “a babysitter and not a grandmother”

I guess she told my husband this during her fit of rage last week because we have rules she needs to follow while watching our son. We are having a talk with her tomorrow and expect this to be brought up and I don’t even know what to say about it. You can … still be a grandmother but have rules in place that the parents wish for you to follow? You can’t just do whatever you want with our baby? You’re not a third parent ?