Seaton's Aunt is a psychological horror short story by Walter de la Mare (1873-1956). It was published in 1923 in the short story collection The Riddle and Other Stories. This story is available to be read online at LitGothic.com.
I had always thought of Walter de la Mare as being a children's author, but he also wrote powerful horror stories. The Kind of Face You Hate has a review of this story and notes de la Mare's influence on Robert Aickman, saying, "if Aickman was influenced by De la Mare, and I've gathered that he was, all the evidence you could possibly need to support that case can be found in "Seaton's Aunt."
It was adapted for tv and shown on Granada'a Shades of Darkness series, though this story isn't included in the DVD set. This episode can be viewed online via youtube:
The adaptation differs quite a bit from the written text. In some ways it tells the same basic story, but in other ways the story it tells is completely different.