***Not sure if this in the right place - mods feel free to move.***

(sorry if I offend any smokers - it's not intended )

My FIL has a big birthday coming up, and DH, DS and I have been invited to the big birthday bash. FIL and his partner live 3 hours away, we're not close with them at all (they've seen DS all of 3 times)... when we see them it's always rather awkward, and we only see them out of family obligation (sounds awful, but it's true ). FIL has invited us to stay with them the night before the big party so we don't have to do too much traveling in one day. That's very nice of them, except FIL's partner is a chain smoker, and she smokes inside the house. In the past DH and I have stayed there, and we always leave reeking of smoke (actually, everything we take with us ends up reeking). We hate it, but out of politeness have never said anything (besides, we only see them a few times a year). However, now that we have DS, I don't want us staying there. I don't want to expose DS to all the smoke (even if she were to smoke outside while we were there it doesn't matter - you can't escape the smell).
DH and I have decided that we won't stay with them, but we need to work out how to tell them without offending. Like I said, we only see them a few times a year, so it seems silly to make things any more awkward or strained. Also, FIL's partner is the type that doesn't see anything wrong (well, unhealthy might be a better word) with smoking, so I can imagine she'll be like "Why, there's nothing wrong with it - my parents smoked around me, blah blah blah". I feel like we can't make up any other excuses, cause this is the second or third time they've asked us to stay with them.

Any ideas how to go about this? DH and I terribly polite people and usually wouldn't say anything - but things change when you have a little bub to think about.