Third-Hand Smoke

I previously posted about contemplating asking my MIL not to smoke the day of watching or holding my baby (due date August 2nd). I spoke with her tonight. She just started Wellbutrin in hopes to decrease smoking, however she stated tonight if it does not work and she cannot quit before the baby gets here she is going to be upfront and let us know that she will not abide by the no smoking day of rule. I've been researching third hand smoke and I think it's so stupid that I have to even have anxiety about this topic, if it's bad for my child then there shouldn't be a question of whether or not my child will be exposed. Basically what should I do? Tell her that if she does not abide by the rule she can't hold or watch her grandchild? That's what I want to do but I don't want to come off as a humongous bitch. Thank you in advance for advice!