Indubitably, the relationship between Roderick and Madeline Usher is bizarre. Key to understanding this relationship is the narrator's description,
I had learned, too, the very remarkable fact, that the stem of 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, and had always, with very trifling and very temporary vaiation, so lain.
So, while the Usher siblings are genetically close as twins, they are even closer genetically to each other than normal fraternal twins since the entire "Usher race" has become inbred. Therefore, in a sense, the entire Usher family is genetically 'incestual.' The narrator further explains,
A striking similitude between the brother and sister now first arrested my attention; and Usher, divining, perhaps, my thoughts, murmured out some few words from which I learned that the deceased and himself had been twins, and that sympathies of a scarcely intelligible nature had always existed between them.
And, it is this aberrant physical, psychological and spiritual union of the twins that in the gothic effect of terror the Usher mansion itself parallels. For, when Madeline collapses from her catalepsy, Roderick becomes so disturbed that his sentience of his twin causes him to mimic Madeline's condition:
...there came a strong shudder over his whole person; a sickly smile quivered about his lips; and I saw that he spoke in a low, hurried, and gibbering murmur, as if unconscious of my presence.
With this "sentience" of his twin, Roderick later declares that he can hear his sister's heart beating:
"Do I not distinguish that heavy and horrible beating of her heart? Madman!--her he sprang furiously to his feet, and shrieked out his syllables, as if in the effort he were giving up his soul--Madman! I tell you that she now stands without the door!"
Hear her, he does; and Madeline who has come from the tomb clutches her brother in a final death agony, killing him. Then, just as Roderick has fallen victim to the fate of his genitive, the house of Usher, too, crumbles to fragments.