what does the narrator learn as he and Roderick gaze on the lady Madeline in her coffin?

The narrator learns that Usher and his Sister are twins.

Usher, divining, perhaps, my thoughts, murmured out some few words from which I learned that the deceased and himself had been twins, and that sympathies of a scarcely intelligible nature had always existed between them.

What does the narrator learn as he and rodrick gaze on the lady Madeline in her coffin