It’s possible that Poe suggests an incestuous relationship between Roderick and his sister Madeline. The argument is based on a few key pieces of info from the story. The first is the fact that the Usher line has never yielded any branches; the line is pure and has always been that way, which some take to indicate in-breeding. Another is Roderick’s admission over his sister’s “dead body” that the two of them shared a special closeness.
Of course, both of these have perfectly plausible, incest-free explanations: the line is pure because only the sons of each generation had children, thus affording no branches, and the special closeness between the siblings refers to the emotional and spiritual connections they share, rather than a sexual one.