Lethargic cat/ No air conditioning causing it?

It is possible your older cat has also been infected. But the vet will tell you for sure. But cats with heavy coats don't do well in hot weather. With your AC out try getting a fan and placing it in an area where she can lay in front of it. Also try putting an ice cube or 2 in her water dish. I have a cat with a heavy coat and I use a fan to help her stay cooler and I brush her daily to remove loose hair. And she gets excited when I put ice in her water dish. She licks the ice. Good luck, hope you babies get well soon.