I don't think you are being "fussy" here. It is natural for one to be protective over their little ones, especially a baby.
On the other hand, I don't think your mother-in-law is doing so deliberately. I would see it as your baby being too adorable, and her cheeks being too "irresistible". Your mother-in-law may have forgotten about the reminder. Try to get your hubby to point it out to her as she is doing so, so that she can see how your baby's cheeks are getting "red" from the tapping.
There are things that our relatives and friends say, or actions they do that we have no control over. Intentionally or not, try to not let these get to you. If possible (especially if the action is making you uncomfortable), try to tell them politely or if someone else is in a better position to do so (like your husband to your mother-in-law), get the other person to do it.
Take care mummy!
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