Speaking as a past smoker, you cannot ask a smoker to quit smoking or don’t smoke unless they are willing to do so. Rest assured, your mil will still continue smoking because my mil is slightly better than yours (during pregnancy).
During pregnancy she will go out and smoke, then stay away from me until the smell of her disperse. Now we don’t live together and when we go and visit her (LO is 20mo), she smokes in her room (the entire house is full of cig smell right after let alone the room) and even when my LO went in her room right after, she don’t bring her out and sometimes she just kiss LO directly after she smoke which I’m very and super annoyed.
I usually tell my husband and my husband will go and scold her or else I will just bring LO away into my SIL’s room and close the door.
If you guys can afford, get a helper or send LO to IFC if you’re really afraid your mil will smoke around baby.