What is Madeline's condition?

We are not apprised of Madeline's condition. From the text, we might infer that she suffers from some sort of depression or other mental illness.

The illness of the lady Madeline had long been beyond the help of her doctors. She seemed to care about nothing. Slowly her body had
grown thin and weak, and often for a short period she would fall into a sleep like the sleep of the dead. So far she had not been forced to stay in bed; but by the evening of the day I arrived at the house, the power of her destroyer (as her brother told me that night) was too strong for her. I learned that my one sight of her would probably be the last I would have — that the lady, at least while living, would be seen by me no more.