Question about URI?

I got 3 new Ragdoll cats 5 weeks ago. I already had 2 Ragdoll cats + a mixed breed cat. The 3 new Ragdolls (mother and her 2 kittens) came together. The two kittens had a upper respitory infection when I recieved the 3 cats. The mother (Ragdoll) never has showed any signs of sneezing or watery eyes at all ever All my other cats have been infected and all are on antibiotics. My question is.........Can a mother of 2 infected kittens become immune to this desease? I just cannot believe that she is not shown any signs of being infected by this URI problem that her kittens both have bigtime

She could have been a carrier of the disease, and since the kittens weren't immune, they caught it from her. Another possibility is that the kittens could have picked it up in transit or from the vet's office.