Yep...what happened here is that even on antibiotics, if you don't separate and isolate the sick cat, the others will still get sick. 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.
This is typical of URI. I have seen so many URI's it ain't even funny!! This just sounds like your cat's got a severe case of it. Your kitty is in the best place right now, where she's getting monitored and getting IV fluids and antibiotic. You sound like a great mom, and you're doing everything possible for your cat. I wouldn't stress too much. Give it a few days for the antibiotics to kick in and for her to get rehydrated.