What confessions does Usher make to the narrator during the final storm?

During the storm, Roderick Usher insists on opening a casement (window) so that the narrator can experience the "whirlwind" outside as the wind changes directions, along with the dense clouds and "huge masses of agitated vapor" that light everything up outside with an eery, unnatural light.
To divert Usher from the storm, the narrator begins to read to him from a story called "Mad Trist." As he reads, the sounds in the story—"cracking and ripping," the shriek of a dragon, and the reverberation of something metallic—are echoed in real life in the house. The narrator is frightened. At this point, Usher reveals that he has known for "many, many days" that he buried his sister alive in a crypt beneath the house. Because of the heightened sensitivity of his senses, he has known for a long time that she was trying to break out of her vault.
Usher confesses this to the narrator, as well as that he did nothing to try to save her from being buried alive: he simply repeats that he "dare not. . . dare not. . . dare not. . . speak." He says he should be "pitied" for having been forced to listen to her all this time. Then, he confesses that he is terrified she will "upbraid" him for his "haste" in burying her. While he is clearly terrified and horrified, he is much more concerned for himself than for his sister.

During the tempestuous storm in the final moments of "The Fall of the House of Usher," Roderick Usher reveals a very disturbing fact to the narrator. Early in the story, the two men buried the "deceased" Madeline, Roderick's sickly sister, in a tomb in a deep vault. Then, while sitting together during the storm, the two men try to ignore strange sounds wafting up from the bowels of the house. At this point, Roderick informs the narrator that he's actually been hearing strange sounds for many days, and that the sounds are actually the result of Madeline's efforts to escape from the tomb. In other words, Roderick reveals they buried his sister alive, and the implication here is he's unwilling to release her because he's afraid doing so will prompt them to engage in an incestuous relationship, just as their ancestors did.
Roderick's disturbing confession comes just before the climax of the story, when Madeline's ragged and bloody form staggers into the room, tackles her brother, and ensures the death of both siblings. It's an example of master storytelling, as the chilling confession makes an already creepy story downright terrifying.