Why, in this sentence do you use "isn't", when "isn't" is a contraction of "is not". the sentence is, "isn't that right? "

When they reached the door of the lonely house and stopped to unlock it, and the cat came to purr loudly, and rub against them, a deserted p***y, indeed, but fat with young robins, sylvia whispered that this was a beautiful place to live in, and she never should wish to go home. the description of the house in this excerpt gives an example of