I'm a bit late seeing this thread.

I'm sorry to hear the kittens tested positive for FELV, but glad to hear the vet is fostering them until retesting. Hopefully they retest negative but if not there are people who specifically take in FELV pos. cats and they may still get to have a home.

Not all cats with FELV or FIV get sick and die. Some live to be quite old with relatively few health problems. It's not very common but it does sometimes happen.

Are your cats current on their vaccinations for FELV? It can be transmitted by saliva and I'm concerned because you mentioned some hissing between the kittens and your cats. Probably a slim possibility but a concern. If they were my cats I'd have them tested in 2 months to make sure they did not pick up the virus. If they are negative (and I very much hope so) then have them vaccinated for FELV after the test. Even if they are already vaccinated I'd still have them tested just in case because no vaccine is 100%. You don't want to take chances with your furbabies.

I always worry when I bring a new cat in the house until the second test comes out negative and all vaccinations are up to date. I have a spare bedroom that new cats are strictly confined to and they don't get to meet the rest of my cats until they've tested negative or until they've gotten their second vaccinations.

I hope things turn out good for the kittens and your own cats. I think it's great the vet was soft hearted enough to volunteer to foster them rather than just PTS for a few bucks. Kudos to the vet.