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She most likely has an upper respiratory tract infection. This is like a "kitty cold."
It is usually caused by a virus and is contagious to other cats but not people. Your other cat had it first and now she has it.
If she is having difficulty breathing, stops eating, or the drainage turn yellow-green she should see the vet for further treatment. The virus can also cause ulcers in the eyes. If she has an ulcer, she will squint her eye and be very sensitive to light. If this happens, she should see a vet as well. Treatment for an eye ulcer would be topical eye antibiotics to prevent infection.
If you bring her into the bathroom, while you take a shower, that will help steam out her nasal passages. You can also keep a humidifier in her room.
You can use nasal saline drops to help thin mucous secretions which will help them drain. Put 1-2 drops in each nostril every 12 hours. You can purchase nasal saline drops at any drug store.
You can give her an antihistamine called "chlopheniramine 4 mg tablets". The dose is 1/2 tablet every 12 hours by mouth if she weighs over 5 pounds and 1/4 tablet every 12 hours if she weighs less than 5 pounds.
If you warm her food in the microwave it will make it more fragrant and help her eat. Cats won't eat if they can not smell their food. Try really smelly canned flavors.
You can also offer her warmed low sodium chicken broth to help increase her fluid intake.
It usually resolves on its own in 7-10 days.
You can read more about upper respiratory tract infections here