what does narrator discover about the brother and sister after viewing Madeline in her coffin? what does this discovery help explain?

When he views Madeline's body in the tomb, the narrator observes that she and Roderick are twins, something that explains their similarities in their nature.

Having deposited our mournful burden upon tressels within this region of horror, we partially turned aside the yet unscrewed lid of the coffin, and looked upon the face of the tenant. A striking similitude between the brother and sister now first arrested my attention; and 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.