In Poe's story he never reveals what the "curse" is on Usher family. Poe writes,
"he (Roderick) then implies that the Usher race is the product of inbreeding, intimating that close intermarriage, if not outright incest, had created a congenital deficiency that may have some part in Roderick's illness."
Some critics believe that they were vampires, some disagree.
"There are, however, many open-ended questions that are not resolved by the story's end. Thus, for example, we do not know whether Madeline intended to kill Roderick in their final embrace or to unite with him. Poe makes no effort to fill in these blanks for us."
For starters, read the earlier enotes thread on this question. For a more scholarly approach, read the article at the second link -- The Vampire Motif in "The Fall of the House of Usher." The author of that piece says:
[C]ritics of "The Fall of the House of Usher" have almost universally failed to recognize that it is a Gothic tale . . . and that a completely satisfactory and internally directed interpretation depends on vampirism, the hereditary Usher curse. Madeline is a vampire -- a succubus -- as the family physician well knows and as her physical appearance and effect upon the narrator sufficiently demonstrate. The terrified and ineffectual Roderick, ostensibly suffering from pernicious anemia, is her final victim.