usually people who are allergic to cats are allergic to their dander...which
would be in every room that the cat was allowed to go in. So having the cat
in an isolated room when she comes over may help reduce her symptoms...the
best choice may be for her to get allergy medication. I'm not sure exactly
how they work (as I don't have any allergies myself)...but there must be one
where she can take a pill before visiting to prevent any symptons of her
allergy.
Hope this helps. Check with the pharmacy or have her ask her doctor.

Isolating the cat when she comes to visit is pretty pointless.. as it is the
dander that is already in the house.

I have found that bathing the cats is the most effective way to reduce the
allergens in your home.  I had a friend who lived locally back when I had
two cats, who was allergic.  I bathed them once a week, and his allergic
reactions went from barely being able to breath for days after less than an
hour, to almost no symptoms after several hours.  If you are seriously
concerned for her, and she'll be spending a good deal of time at your house,
get a kitten, bath it often... she'll get used to it.  I actually have a
foster kitten that loves water now cause she was so messy that I had to
bathe her every day for a while when she was young.  My sister actually
adopted her, and she always "thanks" me for creating a cat who takes showers
with her :)

Thanks to everyone for the advice.

Based upon what I have heard, it sounds like a cat would not be the right
pet for me at this time.  If dander is most likely the issue than isolation
would probably not work.

My sister's reaction can be rather serious (involve breathing difficulties),
and even if she did agree to try taking allergy medication, it might not be
effective. What I really want to avoid is bringing home the cat then having
to return it after it has begun to adjust - not fair to the animal.

Anyway, thanks for the input!

Chris