Thoughts on grandparents kissing baby

Okay so today was the first day my mother in law kissed my 5month old near the lips and on the forehead. For some reason it made me feel uncomfortable. I swear I saw something on the internet about babies breaking out in rashes from other people kissing them ?

Anyways, it’s my mother in law and I feel awkward telling her not to. Like what do I even say and is it ridiculous for me not to want anyone else kissing him on the face?

She’s done it already and now I feel like it’s too late for me to say something now because I didn’t say anything today.