You know what, you have every right for concern & I would be upfront & express those concerns with your MIL. BUT unfortunately if they smoke in the house, even if they smoke outside while you are there, you will still be exposed to the years of yuk that has built up in the house.

I wouldn't go LOL. Is it s visit? or a stay? Could your visit maybe happen at a park? If not, just do your best to spend as much time outdoors as possible & stand your ground on the no smoking around you or your baby. If they feel it is there right to smoke in their home (which I guess it is) but can't see the lack of the respect that shows then just leave the room/house when they do it. Its a hard one & hope they can be understanding enough.