Iain wasn't the one I was worried about putting things in his mouth that he shouldn't. Ada was. I mean that Ada was the threat because she was putting things in Iain's mouth while I wasn't looking. We knew she liked to feed my parent's dogs. Apparently she liked to feed Iain too. And it wasn't anything I would ever expected her to feed him. She had pealed foam sticker letters off an art project DD gave her and started stuffing the bits of foam letter into Iain's mouth while I was in the kitchen. I came back and noticed pink foam on the floor. Then I noticed Iain chewing on something and Ada looked at me like, "What mom? I didn't do anything wrong." Uh huh. Right Ada. Sure.
What would come next? Sticking crayons in his ears? Shoving his head in the toilet? Surely she would find a way to draw on his face with Sharpie markers at some point. Time to activate the eyes in the back of my head.
Moral of the story: Don't expect your older child to behave around siblings because they know better. They don't know better and are testing you to see what they can get away with.