What fact is revealed about the relationship between Madeline and Roderick when the narrator and Roderick place Madeline in this temporary tomb?

The unnamed narrator of the story says that he and Roderick open the lid of the coffin before placing Madeline's body within the temporary tomb, and he notes the "striking similitude"—the arresting physical similarities—between the brother and sister. He says that

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.

In other words, then, Roderick and Madeline are fraternal twins, and this obviously strengthens their bond even beyond that of a typical bond between a brother and sister, because they shared the womb for nine months. On top of this exceptionally close brother-sister relationship, Roderick implies that another level of intimacy exists between them—some relationship that is "scarcely intelligible." I think it helps to recall that "the entire family lay in the direct line of descent, and had always . . ." This, combined with Roderick's statement that the Usher family line dies with Madeline, helps us come to the conclusion that Roderick and Madeline may have had, or were at least expected to have, a sexual relationship that would produce children.

In Poe's "The Fall of the House of Usher," both of the Ushers—Roderick and Madeline—are strange, and they also seem to have an odd relationship with each other. The story implies that the family is so odd because of a long history of incest.
The narrator is visiting the Usher home to see a former schoolmate, Roderick Usher, though he admits, "I really knew little of my friend" because Roderick was so shy and "reserved." Roderick has asked the narrator to come to the house and is hoping that the narrator can provide him with some "solace." When the narrator arrives, already affected by the ominous gloom of the house and its environs, he learns that Roderick is much troubled by "the severe and long-continued illness . . . of a tenderly-beloved sister," Madeline. She is his last remaining relative and they are extremely close; Roderick feels that she will not be alive much longer. Later in the story, it appears that Madeline has died, but her illness can sometimes be mistaken for death. Roderick buries her, but it turns out that she is actually still alive. This contributes to much of the horror and chaos of the story's ending, before the house collapses on the doomed siblings and results in the "fall" of both the physical home and the family line of the Ushers.