Cat Allergy? Is it or isn't it?

My sister-in-law is supposed to be extremely allergic to cats. Such that, whenever my brother and his family come to town to visit, we usually have to meet them in a public place because his wife will have a severe reaction if she even steps into our house for 5 minutes. Although, I have never actually seen her have an allergy attack my brother isn't the type to lie about these things, in fact, he enjoys our cats.
Recently, our mother passed away and, of course, my brother and his family came down to help me with all of the arrangements and so forth. I wanted his wife to be comfortable, so I hired a professional house cleaning service and my home was cleaned from top to bottom. There was a reception immediately following our mother's memorial service and the reception was held at our house and a neighbor's house directly across from us.
My brother and his family spent the entire evening at the house across the street, he came over to our house briefly, but only to say good night as they were going back to their hotel. I made the remark, "How is Kaye doing? I am surprised that she has not had an allergic reaction." You see, the neighbor has two cats and they are even dark colored cats to boot. My brother's response was as follows, "She is fine. She hasn't had any problems whatsoever." I asked, "How can that be, the neighbors have two cats?" He said, "I know, and they were inside the house, too. Walking around among the guests, it must be that Kaye is allergic to a certain breed of cat.
My brother, as sweet as he is, knows nothing about cats. Our neighbor has two domestic house cats, no pedigreeds. The same goes for our cats and our indoor cats are not dark colored. My question is, "How can someone be so allergic to my cats that their throat closes up and they are said to have extreme difficulty in breathing after only 5 minutes in our house, yet they can sit among other people's cats with no apparent problems whatsoever?" Can someone help me with this one?

I have no advice to give but just wanted to let you know that this sounds so familiar. My sister in law pulls the same thing. We have two short hair kittens and since we've added them to our family we have seen fewer and fewer guests due to allergies. I've come to the conclusion that my kitties are too special to be treated in that way. Who needs those guests anyway?......and how could they not love my precious babies???

and I have found that there are some cats I am DEFINITELY more allergic to. It doesn't seem to have anything to do with color, or hair length. I work at the local animal shelter, and the two I am most allergic to now are a 3 yr old white shorthair, and a 4 month old brown classic tabby, both random-bred.
Cats are born with the protein Fel D 1 in their saliva, which is what most people are allergic to. It is not a set amount of it though, which means that some cats have more, and some have less. Moggies seem to have lots, while some breeds, like the Siberian, seem to have way less. Theoretically, it is possible to find a cat in any given litter that you can live with, even with allergies.
It is believed that up to 50% of people are allergic to cats in some capacity, and if you add on to that the number who just don't like cats (through mistaken beliefs, or whatever), it is a lot of people.
I also find that the more time I spend with a cat, the less allergic I am. I foster kittens at home, and with the ones I have now (two sisters, orange tabby and white, one LH, one SH), I was very allergic when they came, but now, 3 weeks later, I am only reactive if I get scratched (then I get hives). I am also less allergic to the LH one. But I like her better too.
I'm not saying your sister-in-law is lying or telling the truth. I'm only saying that it is possible that it is true.