http://www.webmd.com/allergies/guide/dog-allergies
from that site:
Causes of Dog Allergies
You may have heard that some dog breeds trigger allergy symptoms while others don't, or that short-haired dogs are safe while long-haired dogs prone to shedding are not. But on the whole, experts say that isn't the case. In fact, two dogs of the same breed can each give off very different levels of allergen.
It's not the dog's hair or fur that's the real problem. Instead, people are usually allergic to the dander -- flakes of dead skin -- as well as the saliva and urine. So, no matter how long or short the hair, any dog can potentially cause an allergic reaction.
You might wonder why dog dander has such an effect on you. People with allergies have oversensitive immune systems. Their bodies overreact to harmless substances -- like dog dander -- and attack it as they would bacteria or viruses. The sneezing and watery eyes are just the side effects of the body's attempt to destroy or flush out the allergen.
~~~~~~~~~~~~~~~~~~~~~~~~~~
Perhaps by not having been exposed to dogs as a young child, I did have dog allergies as a grown up and still lived with dogs and loved them.. I was worse with cats, which my mother hated from an old superstition about cats smothering babies. I did get a long course of allergy shots. Gradually got better re dogs. I lived with cats too for a number of years, achoo, and also loved them. Had cats and dogs at work in some of my work years besides the dogs at home. That made work quite enjoyable in our studios.
However, I can understand not wanting to have someone's pets in your house - except that you have one already. I can understand not wanting to have hair/dander all about.
A. you could make it a blanket policy to tell any house guests No Dogs first
B. you could put a dog in a crate or kennel outside, or if cold, in the house in a warm room but in a crate.
C. you could insist on that hotel/motel stay. Easy enough.