As the other answer to this question points out, the narrator of the tale realizes that Madeline and Roderick are twins when he sees her in her coffin. This realization heightens the connection between the two Usher siblings and, in a way, makes the possibility of an incestuous relationship (the traditional kind of relationship for the Usher family) between the two even more shocking. For instance, as twins, Roderick and Madeline share an even closer connection than most siblings. Thus, though an incestuous relationship between the two (which is what Roderick is deathly afraid of) is not technically any different than it would be between other siblings, it seems even more unnatural. As such, in some ways the fact that the two siblings are twins emphasizes the decline of the Usher family, as this extremely close relationship heightens (for both the reader and the characters themselves) the unsavory nature of the Ushers' "romantic" patterns.
[Please note: you are not permitted to ask multiple questions. You are only allowed one question, so I have edited your original question to focus on the first question that you asked. Please remember this in future.]
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.