What gives Vera confidence to weave her story about the tragedy in "The Open Window" by Saki?

In
"," Vera gains the confidence to fabricate her tale about her aunt's brothers and
husband after Framton Nuttel says he knows "[H]ardly a soul" in the area. 

When Framton first arrives at the Stappleton's house, he finds himself talking to Vera,
a girl of fifteen, who is the niece of Mrs. Stappleton and a "self-possessed young
lady." She asks Framton if he knows many people in the area, and Framton replies,

"Hardly a soul. . . My sister was staying here, at the rectory,
you know, some four years ago, and she gave me letters of introduction to some of the people
here."

"Then you know practically nothing about my aunt?" the
girl asks.

"Only her name and address."

With the knowledge that Framton Nuttel will not know what is true
and what is not, Vera spins a tale of how Mrs. Stappleton suffered the tragic loss of the male
members of her family, and now delusionally believes they will return. Knowing Mrs. Stappleton
will watch for her husband and her brothers to return through the open window, Vera hopes to
play a practical joke on Mr. Nuttel.

Vera's ruse works so well at blurring
the lines between what is imaginary and what is real that when Mr. Stappleton, his sons, and the
dog return, Framton Nuttel flees in terror, and Vera's joke is complete.