Dear Angela,
The hair may have fallen out because it was matted. The matts do pull on the skin.
However, I would be concerned about ringworm. It does not sound like mange.
You can go to PetSmart or another large pet store and purchase "miconazole" shampoo. Wash her in that. Leave it on for 15 minutes before rinsing. I know that sounds like great fun.
That is what I would do for the weekend.
Keep her away from the other cats. It may be too late...
Hopefully it is not ringworm.
If you have a blacklight you can try shinning it on the spot. The skin will often flourese if it is ringworm. Do not count the dandruff because that always glows.
You are very brave. Yes, if you have help, bathe all the cats. Better safe than sorry.
If you have a dog (s) wash them too.
Maybe you could trim everyone's nails tonight in preparation?
Veterinarian
10 years vet in companion animal practice and humane society