In addition to the weird aspects of the house-- which the unnamed narrator sees as surrounded by unearthly vapors-- the story also deals with a "weird family" of a brother and sister who are the last of their line. It's debatable as to whether Madeleine and Usher share the incestuous heritage seen in other Poe couples, but this would IMO be an adequate explanation as to why Roderick allows his sister to be entombed, even though he knows she's a cataleptic.
I was amused by this section, in which Poe tries to invalidate Kant's theory of the Sublime:
I know not how it was—but, with the first glimpse of the building, a sense of insufferable gloom pervaded my spirit. I say insufferable; for the feeling was unrelieved by any of that half-pleasurable, because poetic, sentiment, with which the mind usually receives even the sternest natural images of the desolate or terrible. I looked upon the scene before me—upon the mere house, and the simple landscape features of the domain—upon the bleak walls—upon the vacant eye-like windows—upon a few rank sedges—and upon a few white trunks of decayed trees—with an utter depression of soul which I can compare to no earthly sensation more properly than to the after-dream of the reveller upon opium—the bitter lapse into every-day life—the hideous dropping off of the veil. There was an iciness, a sinking, a sickening of the heart—an unredeemed dreariness of thought which no goading of the imagination could torture into aught of the sublime.
All of which proves that Poe didn't read Kant's CRITIQUE OF JUDGMENT with any real attention, or he would have recalled Kant's explicit determination that the Sublime can only rise in a viewer of mysterioso phenomena if he the viewer feels safe and removed from any possible consequence. The narrator doesn't feel safe, so why should he feel sublime? But then, Poe was not exactly in Kant's class as a philosopher, so no surprise there.