In "The Open Window" how do you know Frampton Nuttel has never met the Sapplestons before his visit?"The Open Window" by Saki

In Saki's craftily executed frame story, not only is the
main character, Framton Nuttel, taken in, but so is the reader.  Perhaps Saki's greatest
story, "The Open Window" begins with the frazzled Framton Nuttel arriving at the
Sappletons, whom his sister knows, with a letter of introduction which she has provided
him because she worries that he will "bury" himself in his rural retreat meant to be a
nerve cure and not speak to anyone.

As it turns out,
Framton is held captive by the niece of Mrs. Sappleton, and she is a young lady whose
talent lies in blurring the line between imagination and reality so well that poor
Framton is terrified after gullibly believing the niece's tall tale about her uncle and
cousin.  So, rather than working a "nerve cure" for Framton Nuttel, she has unnerved the
man so much that he jumps up and flees without even saying
goodbye.