Of course the tottering and doting old lady appears to be the nicest person in the world, but Dahl gives his own hints about what might be going on.  The overpowering ability of the bed and breakfast sign and its effects on Billy suggests very much that there is more than meets the eye in this little house.  Of course the appearance is one of comfort and pleasure and the warmth that contrasts with the rainy blustery cold outside.  Both the parrot and the dog appear to be perfectly alive until a much closer examination reveals that they are stuffed.  Dahl also leaves the ending entirely out, letting the reader make what they will of the rest of the story, again having given his very clear hints of what is likely to happen.