The disease is poorly understood. A corona virus, that many cats carry in their intestinal tract, mutates for some reason and then causes FIP.
There are then 2 types of FIP, the dry variety and the effusive variety. The effusive or "wet" is a very ugly type and kills cats extremely quickly. The dry can usually be managed for awhile, but eventually most cats sucumb, since there is no cure.
My understanding is that cats younger than 2 years and older than 14 years are most at risk.
I think your indoor kitties are safe and sound. If you introduce a newbie later on, it's a good idea to be cautious about health, where it came from etc.
It's also my understanding that active FIP isn't all that easily transmitted between cats. Supposedly they have to be exposed to the feline corona virus and then in a certain number of cases, this virus will mutate to the type that causes FIP. This isn't true for nursing queens though, they easily pass the virus to their kittens in the milk.
It's a difficult disease that vets struggle with to positively identify. Many times they are only 100% sure when it's too late to doing anything for the cat anyway.
I'm sure there are others on the boards with more in depth information and can correct me if I'm wrong about any of my thoughts on this.