This is an interesting question, because actually, Poe only gives us a physical description of Madeline Usher after she has apparently "died" and the narrator helps Roderick entomb her. Note how the narrator describes her:
A striking similitude between the brother and sister now first arrested my attention... 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.
Thus the narrator discovers that Madeline and Roderick were twins, and we are given the highly ironical description of Madeline in "death" who has all the characteristics of actually being alive. Of course, as we continue to read this terrifying story, we come to realise that her disease that makes her cataleptic without explanation or reason has only served to convince Roderick that his sister has died when she is very much alive. The crucial question of whether Roderick knew this or not is one that the story remains ambiguous about.
For much of Edgar Allan Poe's short story "The Fall of the House of Usher," the character of Madeline is seen by the narrator only sporadically, and Poe offers no physical description of the sister of the narrator's old friend, Roderick Usher. Early in the story, not long after the narrator has arrived at the Usher estate, he is visiting with Roderick when he spies the tormented sister only briefly:
"While he spoke, the lady Madeline . . . passed through a remote portion of the apartment, and, without having noticed my presence, disappeared. I regarded her with an utter astonishment not unmingled with dread; and yet I found it impossible to account for such feelings. A sensation of stupor oppressed me as my eyes followed her retreating steps."
The reader is now introduced to the figure who will be at the center of this macabre story while appearing only intermittently. Madeline's presence is always felt, but she is rarely seen, and that is the way Roderick prefers it. The narrator is very much aware, by now, that Roderick is both protective of his sister and very much ashamed of the legacy with which he and his sister have existed. As noted, however, while Madeline Usher provides an essential underpinning of Poe's story, she is an elusive and presumably highly disturbed character. Indeed, the narrator would never again set eyes upon Madeline while the latter was living, or so he believes:
"I learned that the glimpse I had obtained of her person would thus probably be the last I should obtain—that the lady, at least while living, would be seen by me no more."
Roderick, of course, has fully informed his visitor, whom he has summoned from a distant region, about the curse afflicting the Usher family--the curse, a disease, a madness, that was taking his sister. Soon enough, the narrator is informed by Roderick of Madeline's death. Aiding his friend in the task of entombing Madeline, the narrator provides the only physical description of this character who, we will learn, had been entombed while still very much alive. Describing the process of removing the lid to the coffin in which now resided the body of Madeline, the narrator gazes upon the young woman for the first time:
"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 . . . 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."
Madeline, of course, is not dead. Roderick, so frightened of the mental disorder that has plagued the Usher family, has entombed his sister alive, and Madeline makes yet another appearance, which allows for a wee bit more information regarding her appearance:
". . .without those doors 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."
So, as we can see, descriptions of Madeline emphasize the extent to which mental illness has been permitted to ravage her body--a process certainly aided by the days spent entombed alive. There is, however, another way of approaching the subject of Madeline Usher's physical appearance, and that lies in a detail provided above. The narrator learns from Roderick that Roderick and Madeline are twins. We learn from Roderick that as members of the Usher clan, they both are possessed of the curse that is destroying them. As they are twins, then, we can extrapolate from the narrator's description of Roderick upon their initial meeting after a very long separation:
"A cadaverousness of complexion; an eye large, liquid, and luminous beyond comparison; lips somewhat thin and very pallid but of a surpassingly beautiful curve; a nose of a delicate Hebrew model, but with a breadth of nostril unusual in similar formations; a finely moulded chin, speaking, in its want of prominence, of a want of moral energy; hair of a more than web-like softness and tenuity;—these features, with an inordinate expansion above the regions of the temple, made up altogether a countenance not easily to be forgotten."
This description of Roderick can be used to expand upon Madeline's appearance, though the latter is in a more advanced stage of physical deterioration. The physical similarities that the reader can assume based upon Roderick's mention of his relationship to his sister provides certain details otherwise absent from Poe's narrative, for example, the "nose of a delicate Hebrew model" and the "finely moulded chin." These details provide a fuller picture of Madeline's physical description than do the details the narrator provides upon examining the newly entombed body of this tragic figure.