MIL sucking on baby’s hand

[SilverBirchTree]

My 4 month old baby is going through a phase of constantly sucking on his hands and fingers.

Yesterday MIL said to baby ‘you like having your fingers sucked don’t you?’ and proceeded to put her mouth around his entire hand and suck.

My involuntary facial expression was . Seeing my reaction, MIL said in a baby voice ‘it’s good for my immunity mummy’ and then turned to the baby and in baby talk said ‘you need to build up your resistance. That’s a big word, resistance’ and so forth.

...She then continued to suck on his hands and fingers in front of me....

AIBU or is her behaviour as obnoxious as I feel it was?

I don’t mind people touching or kissing the baby’s hands... but full on coating them in saliva just seems disgusting. And odd. And bloody disrespectful to carry on with if you know the mother isn’t ok with it.

But I’m a first time pedantic mother so happy to be told I am wrong and overreacting.

Any immunity experts? Any MIL advice?

topnan you are spouting shite. No one ever, ever did this. My DM was born in the 20s, she was raised by Victorian parents, we lived in other countries, my kids are almost 30. No one did it.
You're the batshit mil, aren't you?

they now come (expressly invited) for one visit on the weekend and for one dinner midweek

that's a lot! I hope this is a step in reducing visits not an end point?

Pig I thought it was a lot too!! Two visits per week..