There is no reason to euthanize the kittens. However they must be rehomed with full disclosure. closely related individuals share a genetic predisposition to “break” FIP. This is assuming you have knowledge above a guess that the deceased tom is actually their father. The genetic link is being studied at UC Davis vet school (FIP is much more common in certain purebred lines) but is not yet fully understood. The best and only thing that can be done for the kittens is to offer them secure, stress-free homes. As far as other cats in potential adoptive homes, FIP itself is not contagious at all (like cancer the mutation is unique to the individual and cannot be passed on), only the innocuous form, FeCV can be transmitted. It is rare for cats older than 3 to break FIP. Above age 5, it is almost unheard of. So adult cats should be fine. However it is a risk the adopter needs to take on intentionally.
We had to put down a six month old kitten due to FIP in february, her full sib is doing fine at 9 months, but we watch her closely. In some litters every kitten dies of FIP, in others one breaks and the rest do fine – no one knows why.
BTW, the Cornell fact sheet on their FeCV test clearly states that it is NOT predictive of anything. A high titer does NOT predict breaking into FIP, a low titer does NOT predict that cat won’t break. There is NO correlation. Other than to confirm your kitten is a FeCV carrier (which almost all cats in a shelter environment are) there isn’t a lot of “point” to titering FeCV.
However, “sick for months” doesn’t sound like FIP. The symptoms of FIP are elusive and rarely apparent until the disease has become immediately fatal. Cats do not seem to suffer from it very much, eating and playing until nearly their last hours. My kitten was seen by the vet at 9am, brought in with an unexplained pot belly but otherwise eating, pooping, and playing normally… by 6pm she was euthanized with every system shutting down. This is a typical presentation of the more common “wet” form (fluid in the belly). The “dry” form usually presents with neurological symptoms.
It is a heartbreaking disease. My vet says it is hard on the staff too, knowing there is absolutely nothing to be done.