I could have written this post! My MIL has always got her fingers in my children's mouths (and their one now!) She would ask if there's teeth I say yes and where it is but she has to check for herself. It's driving me bonkers! I have now just started handing her a teething ring or something quite bluntly. She's knows I hate it but does it to annoy me! Event with a smirk on her face!
I hear your frustration!