I would think that since your niece is a vet, she would be the best source of answers for you.
Here is what she said to me as a professional:
"Gypsy is a 1 year old female cat who was diagnosed with feline leukemia. Because of this she is hard to place in a home because it is a contagious disease. She would always need to be in a one cat household, remain indoors only. Feline leukemia is a viral disease that can predispose to illnesses later in life but they can live a very long normal life as well. She is extremely affectionate and social. I have dealt with many Felv positive cats and many live a very long healthy life."
That's the advice from my niece but it's always good to hear from people who have actually lived and dealt with cats with the disease...........