In "The Open Window," what is it about Mrs. Sappleton’s niece that causes Framton additional distress?

Mrs. Sappleton's fifteen-year-old niece, Vera, causes Framton Nuttel additional mental distress by telling him an eerie tale about the tragic deaths of her uncles, who she knows will soon return from their hunting trip and walk into the open French window. Vera is described as a self-possessed young lady with a vivid imagination and an affinity for "Romance at short notice." Upon meeting Framton Nuttel, Vera asks him several questions and discovers that he is an extremely neurotic, timid man, who knows nothing about Mrs. Sappleton and is unfamiliar with the region. Vera views Mr. Nuttel as an easy victim and proceeds to fabricate a tragic, unnerving story, which explains why the French window is left open. Vera knows that her uncles will return shortly from their hunting trip and gives Mr. Nuttel a look of horror once he witnesses them walking toward the open window. Framton believes that Vera's uncles are ghosts and becomes overwhelmed with fear. Framton becomes so frightened that he bolts out of the house before the men arrive. Vera's capacity for fabricating stories, her ability to maintain her composure, and her talent for reading personalities allow her to frighten Framton Nuttel.

In short, Vera (Mrs. Sappleton's niece) causes Framton Nuttel additional distress by telling a story that causes "mental excitement." This is exactly the stress poor Framton Nuttel is trying to avoid. He has moved to the country in order to calm his "nervous condition" and is trying to introduce himself to the neighbors without much incident.
Unfortunately for Framton Nuttel, Mrs. Sappleton's neice is the first person he comes in contact with in the neighboring household. Vera is a mischievous liar with a full imagination. As soon as she meets Framton Nuttel and is asked to "entertain" him for a bit, Vera makes up a story about the open window in the house.  
The tall tale concerns Mrs. Sappleton being a "widow" who lost her husband and his brothers in a hunting accident. According to Vera's story, the window stays eternally open in case the men come back someday. The story is a lie. Mr. Sappleton is on a hunting trip at that very moment and returns through the open window. A thoroughly distraught Framton Nuttel runs from the household in fear.