what does usher reveal to be his biggest ailment in the story

Usher revealed his biggest ailment to be the impending death of his sister Madeline.

"He admitted, however, although with hesitation, that much of the peculiar gloom which thus afflicted him could be traced to a more natural and far more palpable origin—to the severe and long-continued illness—indeed to the evidently approaching dissolution—of a tenderly beloved sister, his sole companion for long years, his last and only relative on earth."