She most likely has an Upper respiratory infection:
If she was NOT sneezing a cardiac or bronchial issue could be possible. A cat that is oudoors has a weak immune system so she could not fight it off. It may have turned into Pneumonia or brochitis a this point.
Keep her separated from your other babies as this can be contagious.
All cats carry upper respiratory virus on their own fur. Cats that live together become immune to the carried strain but as soon as the bacterium is exposed to another cat it will infect them. If this baby was exposed to other cats she likely contracted it from them or stress.
Feed her very odorous food so she can smell it and will keep eating. If she gets dehydrated, give him a few droppers of unflavored pedialyte througout the day to hydrate her and balance her electrolytes.
She can have an OTC called clortrimeton - a quarter tablet every 12 hours if she is congested. You can also take him in a steamy bathroom where you have run a hot shower to break it up.For the eyes get Terramycin ointment. some pet supplies have it but call first or order from http://www.revivalanimal.com/
If she developes any colored mucus that means the infection has turned bacterial and will respond to an antibiotic.Some vets script one even if it is viral to prevent secondary infections like pneumonia.
Please let me know how she is feeling. If you get worried I will always be here to help further.
I hope she feels better very soon.
May Gold bess you for taking her in!