In "The Fall of the House of Usher," what does the narrator discover about the brother and sister after viewing Madeline in her coffin?

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Apart from a glimpse of Madeline that the narrator
catches on his first night in the House of Usher, which is enough to fill him with doom,
dread and perturbation, he only really gets to "meet" her when she is lying, supposedly
dead, upon the tressels in the crypt, which rather disturbingly lies beneath the room of
the narrator. So this is the first time that the narrator is able to see her face and
have a chance to consider her appearance. It is then that Roderick shares the
information that you are after. Consider what the text tells
us:

A
striking similitude between the brother and sister now first arrested my attention; and
Usher, now 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.

This special
relationship between the twins perhaps explains the need for them to be reunited in
death at the end of the tale, and the way that Roderick is able to discern that it is
his sister who is coming and making the terrible noise that he
perceives.