Below is some information from the Cornell University web site on FeLV concerning reintroduction of cats. More recent studies have indicated that the virus can't survive more than a few days and that a retest can be done in 30 days. I suggest you err on the side of caution and follow the Cornell recommendation. The full web site is:http://web.vet.cornell.edu/public/fhc/felv.html
To protect healthy cats when a cat is diagnosed with FeLV:
In either a cattery or a multicat household, the most effective procedure is to test by IFA and remove all FeLV-positive cats. The remaining FeLV-negative cats should then be vaccinated and retested every three to six months for the next year, and any that become positive during that time should be removed. The household cannot be considered "free" of FeLV until all remaining cats have tested negative in two sequential tests taken at least three months apart. No new cats should be brought into the household until all the cats already there test negative repeatedly. All new cats should test negative initially, be quarantined for at least two months, and retest negative before being allowed to mingle with other resident cats.
How long to wait until a new cat is brought into the home:
Feline leukemia virus is relatively unstable and will not survive outside an infected cat for an appreciable length of time. The Cornell Feline Health Center recommends a waiting period of at least thirty days after removal of an FeLV-positive cat before a new cat is acquired. Other precautions that should be taken are identical to those described above to protect healthy cats. Thoroughly disinfect or replace the food dishes, litter pans, and bedding that were used by the infected cat. Floors that are covered with tile or other hard surfaces should be cleaned and then disinfected with dilute bleach solution (4 oz. household bleach to 1 gal. water). Thorough vacuuming of rugs, plus the thirty-day quarantine, should be sufficient to eliminate the virus from carpeting in the household.
I am so sorry about your kittens!