You did the right thing to do saving them.

I am usually very skeptic about adopters. I talk with them and ask a lot of questions and sign them over a form, drafted by on e of the shelters. If the cat is not sterilized and vaccinated yet, they must be oblige to do so within a fixed timeframe. It is also important to collect some kind of participation in expenses or to collect the money for prepayment for the vet who will sterilize the cat – people who would not agree – will probably won’t invest in their cat. Nevertheless, you can never be sure in 100%, but you know you gave them a chance.

Can you take them back and find a new home for them? Or at least talking with that family explaining the importance of sterilizing their pets (maybe written articles) and drew their attention to the health of the cats?