How to handle sneaky Inlaw

My son is 9.5 months and we're following the typical baby food guidelines. I've asked my mother in law 100x atleast not to give our baby anything sweet yet. We're still trying to get him to eat vegetables and while wheat stuff. Not to mention I don't know what's in the shit she wants to give him. I'm not here to argue about whether or not it's going to hurt him.

Basically, I don't want him to have that garbage yet. period.

On Christmas (he was 8.5 months) she gave him m&m's and said "oh he can have a couple..." I said- NO he can't. It's hard candy and it's chocolate. She responded "you're gonna have to come stay with Grandma by yourself." I didn't say anything just blew her off but my husband and I both agreed on the way home- NOPE she can't be trusted to babysit. My baby my rules.

She came to visit yesterday and brought doughnuts. I said the baby can't have anything like that yet, he can have puffs or blueberries though.

I walked out of the room- a few minutes later I heard my father in law say "I'm not involved." I came out to her filthy finger in my baby's mouth. My husband let me know he had frosting. I'm livid at this point. Not only was it something I asked her not to give him, not only did she use her dirty unwashed flu season finger, and not only was it behind my back, but my husband knows how it would make me feel and let it happen anyway. His excuse was that it wasn't the time to approach her. Yes. It absofuckinglutely was. He wants to spare her immature feelings instead of mine. At this point, I've told him she's not to be alone with him. And apparently I can't leave the room period. I can't do a quick load of laundry or take a fast shower- I have to babysit the grandmother of my child when she's with him. My husband said today "you know it's the simple fact that you said not to give it to him..." as if he had some epiphany he was sharing with me! I said no shit- that's what I was trying to explain for 45 mins. after it happened.