It's a great gesture on your husband's part to want to help, even with his allergies. If you have some sort of outdoor arrangement for them, that would be great. I would suggest taking them in, and at the same time trying to find them permanent homes. It would be a lot of work, but maybe in the meantime your parents' situation will improve and they could take the cats back too. I know it's hard when you have elderly pets because that dreaded day is always looming and is hard to deal with, but for me, the guilt of putting them in a shelter would far outweigh the sadness of having to put them to sleep.

It's a tough decision, and you and your husband have to determine what is best for you and your daughter first. Good luck and let us know how things turn out.