Friendship is an important theme. The narrator drops everything in order to answer his friend's plea to come "NOW". He has a foreboding feeling as he approaches the house which is in a desheveled state...everything is crumbling, which leads us to the theme of morality. The twins and the house are all connected. The house's state of disrepair is a symbol for the moral, physical, and mental state of Roderick and his sister. Illness is obvious in the two, and the house, which used to be a grand estate, has sunk along with them...alll the way into the ground upon the death of the last two Ushers. So, it is a complete "Fall" of the house and the family whose name the house carries.
The house is then buried, just like Roderick when the house falls in on him. He is, essentially, buried alive as his sister was. If you believe that she was buried alive...was she? Or was it a product of Roderick's madness and insanity? Of course, the narrator swore he heard the scratching sounds as well, but did he temporarily succumb to madness while in the house?
The theme of incest has often been tossed around as the exact relationship of the twins, Roderick and Madeline Usher, isn't made clear. She and he seem to be more than just brother and sister, they are intertwined, connected in ways that may just be the way of twins, but could be more.
Much of the element of the grotesque in this Gothic tale lies in the incest of the Usher "race" that
had put forth, at no period, any enduring branch;...the entire family lay in the direct line of descent, and had always, with very trifling and very temporary variation, so lain.
It would seem, then, that the family had done as many aristocratic families in England so often did: They married cousins (which was not then illegal) and, thereby, made the bloodline so thin--incestuous--that many died or possessed genetic deficiences causing epilepsy and conditions such as Madeline and Roderick possess. The brother and sister are almost the same person because of this thin bloodline, their gene pool is so small that they share many of the same traits and have this bizarre affinity to one another. (the grotesque)
Like twins who share the same genetic code, Roderick senses what is going on with his sister: He knows that she is not really dead when the physician pronounces her so. His art reflects Roderick's knowledge of family history; the lyrics express the demise of a once great "House"/family:
And travelers now within that valley,/Through the red-litten [bloodline] windows, see/Vast forms that move fantastically/To a discordant melody...A hideous throng rush out forever
The "vast forms that move fantastically" is reflective of the grotesque. Poe used the term "arabesque" for his technique of repeating and louping bizarre traits as he does incest with art and madness.