As the narrator and Usher lay Madeline's body in the family vault, the narrator notes several queer details about the corpse before him. First, he notes how strikingly Madeline resembles her brother. Just as he notices this, it seems Usher reads his thoughts and murmurs that he and his sister were twins. Probably the more notable details, however, come in these lines:
Our glances, however, rested not long upon the dead—for we could not regard her unawed. The disease which had thus entombed the lady in the maturity of youth, had left, as usual in all maladies of a strictly cataleptical character, the mockery of a faint blush upon the bosom and the face, and that suspiciously lingering smile upon the lip which is so terrible in death.
Here, the narrator notes that as a result of her disease (which, though undiagnosed by doctors is one that causes her catalyptical seizures, or physical fits which leave her entire body rigid, as if she is dead) she has died with a faint blush upon her bosom and face, and seems to be smiling. A normal corpse would of course typically be devoid of color, as the blood would no longer be pulsing throughout the body. Also, it is particularly strange and eerie to the narrator that Madeline seems have died with a smile on her face.
All of these details are particularly important when you realize that they are foreshadowing a most bizarre and ultimately creepy end to the story. Madeline, at this moment, is not actually dead. She is simply in the throws of such a seizure, which is apparently severe enough to have slowed her heartbeat and breathing to an undetectable state. Note that at the climax of this ghost story we see:
...there did stand the lofty and enshrouded figure of the lady Madeline of Usher. There was blood upon her white robes, and the evidence of some bitter struggle upon every portion of her emaciated frame.