After Madeline Usher's death, the narrator helps Roderick to place her body in its "temporary entombment." He says,

The body having been encoffined, we two alone bore it to its rest. The vault in which we placed it [. . .] was small, damp, and entirely without means of admission for light; lying, at great depth, immediately beneath that portion of the building in which was my own sleeping apartment.

Once they put her coffin down, they open it to look at her face, and the narrator is struck by the similarity in looks between brother and sister. He learns that Roderick and Madeline are twins and that they've always had a kind of special sympathy between them (of the kind that twins often seem to share). They note that she has retained a bit of pink in her cheeks as well as a slight smile, and they replace and refasten the lid, securing the iron door and leaving the room.

After some few days, the narrator notes a change in Roderick's demeanor: he seems to suffer from "extreme terror." About a week after Madeline's interment, Roderick comes to the narrator's room and casts open a window to the storm, quite agitated. The narrator shuts it, draws his friend away, and offers to read a "favorite romance" to Roderick. He hopes that it will assist his friend to calm down, to "find relief" from his hypochondria. No such luck.

The narrator helps Roderick Usher move her body to another part of the house and secure it in a "vault" of sorts.  The narrator and Usher believe that Madeline has died when they do this, but the reader finds out later that she, in fact, is NOT dead.  When she appears at the end of the story, she falls upon her brother and he also dies.  The narrator manages to escape from the house before it crumbles.