Telling people not to put their fingers in baby's mouth?

Especially when it's the in laws?? But my dad does it too.. I know "it's your baby just tell them not to". But that doesnt always work. I dont want to be stern and come across as overreacting to the situation but it does bother me. And I know baby has to get some germs to build immunity for later in life so I'm trying to just let it go but it's hard. Any tips/advice?

In response to comments. My baby is teething, so they do it "because she wants something to chew on" or "to feel for teeth" or "she likes it". Especially grandma in law, my baby wont even be acting bad at all and she will reach over and say "I bet she wants to chew on my finger" and then if I pull baby away she acts offended. The problem is I feel like if I say "I dont want anyone doing that" they will do it when they are babysitting and I'm not around. And not letting them babysit is not an option. My dad will try to find a teether but if he can't then it's his finger. My in laws dont even try to look for a teether, they just pop their finger in her mouth like its normal.