I don't even know what to do

Carly
I should probably start by giving you a little background. My husband and I had our first son in June, the first grandchild on both sides. Both of our mothers are goo-goo ga-ga over him and constantly want to be watching him. However, i don't trust my mother in law to watch him AT. ALL. Here's why. 
She's one of those persistent types- the one who knows the best things for MY child and usually those things don't align with my wishes. One thing we don't see eye to eye on (among many...but that's for another day) is that she insists constantly that I should be feeding my TWO MONTH old son food. After I told her we weren't introducing foods until 6 months, she went out and bought rice cereal and keeps it on her counter tops to taunt me. 
Tonight sent me over the edge. I was there visiting her and my father in law, and she tried feeding my son mashed potatoes.....with me in the room! She first tried to sneak it to him and when she saw I was looking said "don't look!" and proceeded to stick a spoonful of mashed potatoes near his mouth. I had to yell "NO!!" THREE times before she stopped trying. To make matters worse she started talking baby talk to my son and saying things like "your mommy is so mean. You don't want any more milk...you need food!" Like what. The. Heck?!?! How ignorant. 
We've been in multiple arguments with her over this and she still keeps trying. It's like since we told her not to, she's gotten even more persistent and wants to "win" so to speak. 
I don't want her watching him anymore. We've had her watch him a few times and I cringe wondering what went on when I wasn't looking, especially with how bold she was tonight with me in the room! Ugh. It's just a hard spot to be in. I don't want to ruin our relationship but it's getting harder to sit back and be passive these days. My mama bear instincts are in full force! What do I do?!???