Feline leukemia is spread by direct blood-to-blood or saliva in a wound contact. Cats can't get it or spread it by sniffing or licking or even by grooming each other unless saliva from an infected cat gets into a wound or sore on the cat being groomed.
I've known of feline leukemia positive cats, even ones who died of the disease, living with other cats in a household, and none of the other cats ever developed the disease.
A major cause of feline leukemia in kittens is being born to an infected mother who got the disease from being mauled by tomcats during mating. Tomcats often spread the disease via their fighting.
For your peace of mind, get your cat vaccinated for feline leukemia and the other common cat diseases, if you haven't already.
I work at a vet hospital.