GL with dealing with this. Family can always make things so difficult.
I stayed with my sister and BIL when I was in Sydney in August and I asked them if they could not smoke in the house while I was there. They stopped smoking in the house a few weeks before and it was the first time I've been there and haven't been hit by a wall of smoke smell as soon as the front door was opened.
I think the suggestions to educate your inlaws on the dangers of second hand smoke are good. Can you send them information beforehand, just to explain why you are asking for their cooperation? If you make it all about your baby and the health problems rather than about you, then I don't see how they can justify being upset if you need to set some restrictions.
