Rodrick truly believes his sister to be dead, so he and the narrator entomb her. The reason behind her two week timing is to heighten the suspense. When the two men laid her to rest, she had "a faint blush upon the bosom and the face" and a suspiciously lingering smile upon the lip" as they placed her in the tomb. Because the reader sees this along with the narrator, we realize that this is foreshadowing. We know that she is not dead. That is very characteristic of Poe's writing as well. We wait for Madeline to once again appear, and two weeks is a long time to have a person buried below you--and not yet officially sealed off. That was what added to the horror of the story.