My Nan smokes BUT she is more than happy NOT TOO even in her own home when people visit.
We let her know in advance that we are coming so she starts smoking outside say a week before, makes sure the house is Vac'd and dusted, washes her hands and brushes her teeth before we get there.. i'm almost certain she changes her clothes too.

I would strongly advise that you mention to MIL the issues with bub, mention that bub will be very sensitive to smoke etc and for that reason when he/she is born only parents will be allowed to nurse or touch him/her (lie and say hosp rules).
Also mention that once out of hospital the Dr's have told you to be very careful, bub is NOT to be around smoke or other 'allregins' (prob more believable if it's not JUST smoking KWIM) and should they want to visit they will need to be wearing clean clothes and cannot smoke near him/her.

Not suer if that will help, just mention you have bubs best interests at heart and these are the rules.