Just an idea, but could it be well water and something that's in it that they have become used to?
I wouldn't take bleach along, but I'm sure there's somewhere you could go get it after you get there. The wipes are a good idea. I wouldn't hurt their feelings and just start wiping when I walked in, but since there are other family members there, maybe you could all go in and clean top to bottom as a holiday surprise. Just do it without upsetting them. Heck even get someone to take them out for a day while a few of you go to town in there.
I wouldn't take bleach along, but I'm sure there's somewhere you could go get it after you get there. The wipes are a good idea. I wouldn't hurt their feelings and just start wiping when I walked in, but since there are other family members there, maybe you could all go in and clean top to bottom as a holiday surprise. Just do it without upsetting them. Heck even get someone to take them out for a day while a few of you go to town in there.