What does Roderick tell the narrator about his sister Madeline?

Roderick is never specific. He does tell the narrator that her illness was beyond the physicians’' practice, " A settled apathy, a gradual wasting away of the person, and frequent although transient affections of a partially cataleptical character" This means that besides wasting away, she also suffered seizures. The narrator only sees fleeting glimpses of Madeline which makes us wonder if she really even exists.