What does the narrator discover about the brother and sister after veiwing madeline in her coffin?

While viewing Madeline in her coffin, the narrator comes to the realization the Usher and Madeline were twins.

The exact similitude between the brother and sister even here again startled and confounded me. 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.