This sounds so familiar! We got a new kitty the Sunday after Thanksgiving and she had an URI, then last Friday our older kitty came down with it. Remember, too, that the germs will circulate to some degree through the heating/AC system in your house.
Separate the currently sick one from the others and allow no exposure to the sick one. Wash your hands with antibacterial hand soap before & after visiting with the one in isolation. Make sure the sick one is on some kind of antibiotic. If possible, wash the food bowls and water bowls in antibacterial dishwashing liquid daily and use HOT water. Wash any towels or wash cloths used in the sick room frequently.
To help with the congestion, you can use a humidifier or vaporizor to moisten the air. I've also been putting a couple of drops of eucalyptus oil on a washcloth near where the kitties are sleeping to help clear up the stuffiness in their noses. My vet also recommended putting just a little dab of Vick's on the tip of the nose to help open them up. (We have been using Baby Vick's).
The best thing is to remember to wash your hands and any other exposed part of your body that the sick one sneezes on or rubs on.