Hi babyjaden2004, welcome to the forum!
I can relate to the parents smoking problem--maybe I should give you my dad's phone number!! He smoked in front of me for nearly the first half of my life...after his near fatal heart attack in 1995, he was VERY apologetic about it, realizing what difficulty it must have been for me to breathe, but I told him I forgave him for that.
I'm curious which parents are smoking? Yours or your husbands? Depending on who's they are, I would feel they're more likely going to listen to their child about how and what to do around their grandchildren. (in some cases, it may be the opposite, so whomever they listen to ought to talk to them). This is where the need to have a serious discussion may come about. (Can't be helped)
Indicating that one important point is that the child has complications to breathe, and bringing "added" problems to it complicates it more. Perhaps, as a suggestion, telling them the difficulties involved with your 15 month baby, (exagerating more than needed may give leverage), may give some foundation to insist they do not smoke within a certain time before entering your household. You may offer that by not following these rules, inviting them would become difficult. Stressing that though you love them very much, your love for the child and the child's health and well being would overrule such decision to invite the parents without them following your wishes, and requests.
As someone who's experienced having parents who didn't know the problems with smoking around a CF child, I hope this gives some help, ideas, and takes some of that frustration off.
This forum is "dedicated" to those who are looking for help, suggestions, or even just to "vent" sometimes. I think you came to the right place!!
Good luck with your endeavor...