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.