Poe wants the reader to also consider the Gothic literary element that is consistent with all Gothic literature, which is the inevitability of fate. In other words, not only are the Ushers unfortunate because of their own choices to remain isolated from the rest of the world, but they may very well may be a doomed family. This form of doom is not just manifested only because of their genetically challenging disease, but because they are simply haunted by something that is more powerful than themselves or anything human. Adding this element would complete the whole idea of what is wrong with them. It has to be something both physical and ethereal for it to add to the terror of the tale.
Just as in nearly every other short story by Edgar Allan Poe, "The Fall of the House of Usher" is meant to revolve around a pervading presence of evil. Something evil lives and lurks within the house itself as well as within the family of Usher. A large part of this evil, is incest.
The quote given in the first post is the first (although subtle) clue that Roderick and Madeline derive from a lineage of incest. Later, however, it has been suggested that these twins have an unnatural relationship with one another as well. Madeline is described by the narrator as "a tenderly beloved sister—his sole companion for long years." Some have pinned this point against Roderick's nervous disorder which "displayed itself in a host of unnatural sensations," and concluded that the idea of incest is not merely suggested, but conclusive.
Certainly the narrator detects something evil, even to the point of unnatural within the house.
As the narrator states,
...the Usher race, all time-honored as it was, had put forth, at no period, any enduring branch: in other words, that the entire family lay in the direct line of descent...
These passages, in addition to others, lead the reader to believe that the true problem with the Usher family is caused by a form of mental illness that is passed down from generation to generation. In addition, it is revealed in the story that the symptoms of the disease worsen with each passing generation. The fact that the Usher family has not managed to intermarry with other families and, thus, dilute the potency of the gene or environment responsible for the illness plaguing the family, has led to its ruin.