I concur with Dr. Karnia, the issue is likely an upper respiratory viral infection such as herpes or calcivirus. Over the counter cold medications for humans are very dangerous to cats, and should never be given. You can talk to hyour veterinarian about l-lysine, a safe oral antiviral supplement which can be helpful, and can be obtained OTC.
Good luck,
Kimberly Coyner, DVM DACVD
Upper respiratory infections are very common in cats especially kittens. Your kitten either exposed your older cats to a new virus or the stress of bringing a new kitten into the house reactivated a virus they already had in their bodies. This is a very common situation.
The best thing is to have them checked by your veterinarian to make sure they are not dehydrated. As long as the eye and nose discharge is a clear watery discharge, the eyes are not swollen or held shut, they are eating and drinking well, and are doing their usual daily things, you should not need to worry. The best thing is to keep them hydrated by encouraging them to drink water or add water to their food. Watch them closely though for any problems. Older cats are more susceptible to dehydration or not eating well.
Good work on having them up to date on their vaccines. It probably helped this be a mild illness and not anything more severe.
Judy Karnia, DVM
Scottsdale Cat Clinic