Advice, please

Laura • SAHM

I need some advice. I do NOT like my children drinking out of anyone's drink unless it's mine or my husband's. It just grosses me out honestly because I don't know everyone's hygiene. My mother in law has a bad habit of just letting my child (he is only 2) to drink from her drink. It drives me nuts! It has happened only a few times and she is super sensitive so I have not just came right out and say it, I'm looking for a nice way to say it where she gets the point but doesn't sound offended. I'm afraid she'll be offended anyway, because that's just how she is. But if anyone has any advice on a kinder way to say it other than please don't that grosses me out, let me know!