I made it a point with my mom and MIL before my son was born that unless something is dangerous and they can back it up, to stay out of how I parent my baby. My MIL is great, but I've gotten aggravated with my mom a few times. I'd just remind her again of the boundary and tell her something along the lines of "I understand that you're trying to be helpful, but it upsets me when you tell me to do x,y, and z to care for my child. I am babies mother, and I already do those things without a reminder."
I feel the exact way, like my MIL doesn’t think I know how to care for the baby. And yes it does feel like she thinks the baby is hers but I just suck it up since I need the childcare… I hope one day I can get the courage to speak my mind. It is frustrating but I feel good knowing she is also monitoring these things. I’m not sure the older generation thinks much about it, that they just are naturally inclined to assert themselves and their opinions?