Why is it important to the story that Framton doesnt know anything about the Sappletons?
Because Framton does not know anything about the Sappletons, Vera is able to play a joke on him. She tells Mr. Nuttel that Mrs. Sappleton's husband and his two brothers went hunting one day and never returned. Vera continues by saying that Mrs. Sappleton now leaves the window open in hopes that they will come home one day through the same window they left. Framton believes her tall tale and when the men do return, he flees with fright.
It is important because this allows Vera (whose name, ironically, is the base of the word 'truth') to trick Framton. She is able to tell this elaborate story about her uncle and his disappearance and Framton believes the entire thing because he doesn't know any better. Her story causes him to run from the house in fear at the end when the uncle returns from hunting.