It only takes one little
sneeze from the kitten for all the
germs go out to the air, allowing your older cat to easily get ill. Next time you have this experience, isolate the sick kitty. Yes, it is URI, and I hate to say some cats don't make it, but it sounds like everything that needs to be done is already being done, so it is just a matter of time before the kitty bounces back.
I wish you all the luck in this world. Please keep us updated on your cat's progress.