what is the symbolic meaning of roderick usher and madeline usher and their interaction at the end of the story? What does Poe argue for?

There is a clear incestuous undertone to the relationship between the brother and sister. Without spouses they live together in the great family home, each of them wasting away within the building's dark rooms. The Narrator describes the strange qualities of the Usher family--that it never has put forth "any enduring branch," that "the entire family lay in the direct line of descent." The implication is that incest is the norm for the Ushers, and that Roderick's and Madeline's strange illnesses may stem from their inbred genes.