What details about Madeline's corpse might be important?

The narrator notes that Madeline really resembles her brother. He also notes:

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.

Unlike a normal corpse, Madeline seems to have color and is smiling. Is Madeline really dead?

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.

She appears dead, yet there is a certain life in the blush upon the bosom and the face and the smile on the lip. It is almost as if she is not dead.