I really like this quote, especially as it is applied to "The Fall of the House of Usher." It is clear that you could easily refer this quote to the somewhat parasitic relationship that exists between Madeline and Roderick. From the very beginning of the tale, the narrator establishes a link between them, and not just concerning their status as twins. Both are described as half-dead human cadavers, with their mysterious diseases having wasted their flesh away. If you look carefully at how the narrator describes both of them, it is clear that he struggles to connect them with any human form. Note what he says about Roderick:
I could not, even with effort, connect its Arabesque expression with any idea of simple humanity.
The sight of Madeline strikes the narrator with a feeling of "utter astonishment not unmingled with dread," which the narrator finds it impossible to account for. Clearly, the fact that Roderick and Madeline live so far away from the real world in an isolated, gloomy house, which again is linked to its owners by its description of rotting decay, has helped the siblings to prey on each others fears and concerns with tragic consequences.