No, I don't allow too and it may cause the baby to fall sick. Maybe you can forward the article that talks about baby falling ill because of a kiss from the relative? It can get very serious especially if she is carrying some viruses which do not affect her but can be harmful to your baby. Tell your husband too... maybe it is more effective if it is your husband who tells your SIL?
Hahahaha just tell SIL? But then again, she doesn’t have ill intentions.