My cat has leukemia but..?

She isn't eating, not going to the bathroom, and she keeps sneezing and is occasionally drinking water. I know shes very sick and I don't want her to suffer so my dad says we should put her to sleep. But the thing is, I just wanna know if hes lying to me due to the fact treatment is probably expensive. He said the vet said she has had it for about 2 to 3 years now. Is it possible that shes had it for 2 to 3 years? She never showed any signs of sickness at all and all of the sudden shes bones and trying to hide and sleep. And is there any way I can make her feel better? I'm trying to console her and caress her but she doesn't want to be touched (which is really weird and heartbreaking to me, shes acts like a baby) and she wants to hide in tight places. Please help