Pushy Parents

My MIL is constantly trying to get me to put cereal is my daughter's bottle. I know not to and my daughter is only three months. She keeps saying what do "they" know. Saying that the experts don't know MY baby. Does anyone else deal with crap like this? And if so, how did you deal with it? I'm close to snapping on her because it's really annoying being told how to care for my child.