Why was Framton Nuttel's sister worried about his going to the country?

In addition to having a nerve problem, or neurosis, Framton Nuttel appears to be a shy, reclusive man who spends much of his time alone. His sister knows him well, and she has presented him with a batch of letters of introduction in order to try to force him to get around and meet people while he is staying in the country for an attempted nerve cure on doctor's order. 

"I know how it will be,"...

In addition to having a nerve problem, or neurosis, Framton Nuttel appears to be a shy, reclusive man who spends much of his time alone. His sister knows him well, and she has presented him with a batch of letters of introduction in order to try to force him to get around and meet people while he is staying in the country for an attempted nerve cure on doctor's order. 

"I know how it will be," his sister had said when he was preparing to migrate to this rural retreat; "you will bury yourself down there and not speak to a living soul, and your nerves will be worse than ever from moping. I shall just give you letters of introduction to all the people I know there. Some of them, as far as I can remember, were quite nice."

Framton's sister seems much more confident and aggressive than her brother. She worries about him because she doesn't believe the countryside will do him any good if he just hides in a rented room somewhere, as she feels sure he is likely to do. This is an effective way for Saki, the author, to plunge his shy, nervous viewpoint character into an English country household where he doesn't know anything about the people upon whom he is calling. It is, of course, essential that he should not know anything about the Sappletons in order for Vera's story to have the effect she desires. She makes sure Framton knows nothing about her family and virtually nothing about anybody else in the region by asking him just two questions.

"Do you know many of the people round here?" asked the niece, when she judged that they had had sufficient silent communion.

"Then you know practically nothing about my aunt?" pursued the self-possessed young lady.

Her leading questions when she first meets him are intended to inform the reader, along with Vera, that Framton is the perfect patsy for her practical joke.