Sick cats hereditary?

Question
Hi, my question is about three cats I had/have who all have/had the same symptoms.  Two of them died from it.  It first started five years ago when the 1st cat started throwing up a lot.  She would eat then throw up everything then go back and eat more as if nothing happened, but not everytime she ate.  Sometimes she would chew on plastic to make herself throw up.  Eventually, she suffered from constant diarrhea and over many months she slowly lost weight until when I would run my fingers down her back I could feel every individule vertebrae she was so thin.  It was as if she just wasted away then she died.  I had taken her to a vet but he just gave her a shot and said she may be allergic to cat food.
About two years later, her niece came down with the same thing and died, they had never been in contact.  Now, I have a four and half year old cat who has the same problem but he is still alive.  He is the first cat's nephew, 1/2 brother to the 2nd cat.  He seems to be barely holding on for just this past week and has lost a considerable amount of weight.  The vet says the same thing about being allergic to his food.  I was thinking of taking him to another vet for a 2nd opinion but I didn't want to spend loads of money to have another vet tell me the same thing.
I love my cats, please help!  I appreciate any info. you can provide.  I was wondering also if you know if there is a website I can go to to type in my cats syptoms and see what he may have.      
Thank you so much.

Answer
Kimberly,

By all means, go to another vet.  I cannot believe that's all he could say is that perhaps they are allergic to cat food!  That is crazy.   Your cats obviously have something very serious going on.  They need to be tested for leukemia.  Yes, it can be passed on and is very contagious.  There are other problems that blood work could also show.  Please, run....don't walk, to another veterinarian as soon as possible!!  I doubt another vet will tell you the same thing!

God Bless,
Becky