I would suggest that before you return the chis to your mom's house you make sure the carpets are cleaned thoroughly and all other floors are also cleaned. Simple Solution makes a carpet cleaner that will help remove the old odors. I would probable use a squirt bottle with regular Simple Solution (or similar product) on the baseboards and anywhere I think they might have wet before. It will take a couple of days to dry and really eliminate the old smells, but it will keep the chis from wanting to cover their own old urine.

I have to keep a bottle on hand all the time. Samson likes to water my plants.

Mary