OP, I could have written your exact post when I was expecting my daughter 4 years ago. Both of my in-laws smoke and I freaking HATE it. (Well, okay, I don't much care for them anyway, but the fact that they smoke really doesn't do much to endear them to me.) Like you, I was really concerned about the third-hand smoke issue when my daughter was a newborn (see, for example,
http://www.mayoclinic.org/healthy-living/adult-health/expert-answers/third-hand-smoke/faq-20057791).
Thankfully my in-laws never stay with us, but if they were in the habit of doing so, I would have used the "I don't want anyone but my own mom staying with us" line to get them to stay in a hotel. Having a newborn is stressful enough. You do NOT have to accommodate people in your own house that you don't want to be staying there. Your husband should back you up on this, and be the one to talk to his parents about it.
As for my wish to have my in-laws go through a complete decontamination process after every cigarette (shower, wash hair, change clothes, brush teeth) -- well, obviously that wasn't going to happen. They washed their hands before holding the baby, of course, and then I would practice deep breathing and try to focus on the fact that they were bathing her in love (as opposed to toxins). At her first peep, I would whisk her away to nurse or check a diaper or just because I am her mom and I wanted to hold her. And I didn't care what they thought. The bottom line is that I resented them for putting me in the position of being forced to let my daughter get that close to known cancer-causing chemicals in her first days of life. And yes, I know life entails risk every day (before someone asks me if I dared to drive home with my newborn in a car, THE RISKIEST THING YOU CAN DO!!!!) -- but it's not like they even made an effort to use an e-cigarette or chew nicorette or whatever. I think it's bullshit that a smoker's addiction forces their non-smoking loved ones to compromise their OWN health.