Being "allergic to cats", may not be simple. Owning a cat can expose a person not only to the cat, but to cat litter, to cat food, to cat blankets, to cat toys etc. Any of which may promote an allergic reaction.
Also an outdoor cat could bring allergens indoors on its fur.

It is therefore not as simple as testing the person for the common cat allergens, so the doctor is right to say there may be some reaction to a cat if you brought it home tomorrow.

I would guess the allergist from her experience is of the opinion that whilst getting rid of pets in an atopic patient household is perhaps taking things a bit too seriously unless there is a good reason to do so, BUT adding pets to the pet free household of an atopic patient is perhaps asking for trouble.
JMO

Obviously opinions diverge a lot. I'm really not sure what to do here. I'll probably just visit a couple more breeders and spend some time there and see what develops although I'm seriously considering giving up on the idea altogether since it's obviously causing too much troubles and doubts. Maybe I'll just have to wait until I have my own house, oh well... Someday!

Anyway, I appreciate a lot all of your suggestions and comments you have been really helpful. I'll keep you updated.

Thank you so much!

Junglerose brings up a good point regarding the other cat stuff your sister may be allergic to. I am allergic to dust and cat litter does bother me, though only when I clean the boxes.

Some allergists immediately jump to "NO PETS!!". I can tell you that what sold me on allergist #3 was that he never told me to get rid of them. His attitude was that pets are part of your family and as long as you aren't having a deadly reaction, they have more health and happiness benefits for you there than not.

I would try to find a breeder that will take the kitten back if there is a problem. Honestly, if a breeder won't agree to take a cat back if there is a problem, that tells me that breeder doesn't really care about their cats, a big red flag, in my opinion!

I would also find a breeder that will let your sister put in some free labor while you and mom visit with the cats. I don't know of any cat person that would say, "oh no, please leave the cat poop in the box, it is so fragrant, and that litter strewn all over the floor, I like the way it feels on the bottom of my feet while I am making dinner! Leave it there!" That way she would be exposed to the most allergic parts of cat ownership, in an extreme situation. Or have her volunteer at a shelter and do those sorts of jobs. It would expose her and you to the dirty parts of the job and she would really be able to judge how she'd do.

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Lollo