It begins of course with the crumbling castle of a once-great family, with both edifice and inhabitants being the eponymous, falling "house." The last members are a brother and sister, Roderick and Madeline, both of whom are in poor health, physically and psychologically. Because of their specific illnesses, Roderick lives in fear of harming Madeline and unwillingly becoming the gothic nobleman who oppresses a young woman.
"Usher" isn't one of my favorite Poe stories, mostly because of how fantastical it is. I'm all for supernatural elements like ghosts and witches, but those are at least grounded in reality and I understand the rules about them. Poe goes too far when he mystically ties the siblings' well-being to their home, with no real attempt to explain why the house might be alive and sharing the fate of the family. It's great symbolism; I just have trouble accepting it as a plot point.
I've never seen Roger Corman's 1960 movie adaptation, but I understand that it fixes some of my problems. It disconnects the family's supernatural relationship with the castle while having their dysfunction bring about the destruction of the building in other ways. It also grounds their relationship with each other by giving Madeline a fiancé and having Roderick (Vincent Price) disapprove enough that he's willing to do horrible things to his sister in order to prevent her marriage.