Why had Poe made Roderick and Madeline twins and not just brother and sister? What does Madeline seem to represent? What does Roderick represent

In some ways they seem two parts of the same body, as twins often do. When Madeline begins to decline, so does Roderick. As she dies, he begins to feel he is dying. They seem to have a "twin" connection which relates to the ending when the house falls down around them, dead together for all eternity.