Why doesn't my cat use the litter box?

It seems that stress is at the bottom of many problems such as you describe. Possibly your cats underwent a lot of stress when they had to live elsewhere, in a place that was not up to what they were accustomed to. Take a look at DearTabbyTheAnswerCat. They have an excellent article under "Cat Care" on "Litter Box Problems." It tells the many reasons for such problems, and how to solve them. Hope this is helpful!