Being polite

I need a polite way of saying to my MIL to stop taking my baby from my arms when we go anywhere with her or just around her house. My OH seems to think she can because she's his grandmother. The other day he was sitting nicely on me and without saying anything she just took him from me. I have no idea how I managed to bite my tongue but I did. We've got a family event with their side of the family coming up soon and I know it'll happen all day. He's only 2 and a half months old I'm not ready to have him gone for too long.