This is a really tough situation. The Feline Calicivirus is a common upper respiratory pathogen. By this point, probably all the cats outdoors have already been exposed to the virus. Incubation is about 2-6 days. If any of these cats have been vaccinated with the "distemper"/FVRCP vaccine, then they should have some protection against this virus. Many cats can obtain the virus and not have any clinical signs and some may be hit harder. Some can also become carriers of the virus and shed the virus intermittently for months or years. This is going to be very difficult to control in your feral colony but it is possible these cats may not become clinically ill.
There are a couple things I would recommend. If you bring the ill cat indoors, you are putting your indoor cat at risk, cat even if they are kept separate. But if there is anyway you can keep the ill cat segregated (in a large carrier in a garage, or a large kennel, etc.) this may help prevent spread of the disease to the outdoors cats and also allow you to better monitor her progress as she fights her illness. I would also have the kittens vaccinated as soon as you are able. They will have some protection against the calicivirus that they obtained from their mother but this will be waning as they get older. When kittens obtain this virus, it often hits them harder and they can become more ill. I would also encourage you to keep things as clean as possible. Use a dilute bleach solution on all bowls etc you use for feeding and if you can it may be a good idea to set up multiple feeding stations as well. The virus can also be shed in the stool so if any of these cats are using litter boxes, clean the boxes of stool as frequently as possible and bleach the boxes when you can. I hope this helps. Please let us know if you have more questions and I hope that you will just have this one ill cat!
More...