My Sister In Law Puts Her Finger In My Baby’s Mouth – Leo and Rebecca Buzz Question

Leo and Rebecca Buzz Question...My man's SISTER comes over to see our 4 month old baby and SHE PUTS HER FINGER IN HIS MOUTH to soothe him as a pacifier!! I was shocked when I saw her do this and was like WHOA! After she left I told my man I'm not having that..and he said it's no big deal..she was just trying to comfort the baby. I FEEL LIKE I NEED TO CALL HER OUT IF IT HAPPENS AGAIN...what do you think? Gross!!!!