What does the narrator learn about the relationship between Usher and Madeline after her death?

The narrator learns that Roderick and Madeline were twins, and that there had always existed some kind of intangible bond between them.

As we placed the lady Madeline in this room of horror I saw for the first time the great likeness between brother and sister, and Usher told me then that they were twins — they had been born on the same day. For that reason the understanding between them had always been great, and the tie that held them together very strong.