As the other two answers have mentioned, the Usher family is trapped within a sinister and vicious cycle and falls due to the inability of its descendants to break free of that cycle.
Incest seems a likely candidate for the perpetuation of the madness and destruction within the Usher family. However, the house itself takes on a malicious character. Roderick claims to be influenced by the house's bad vibrations so to speak. Those who remain in the house appear to be consumed and destroyed by it. This is an apt metaphor for being trapped by tradition.
So while incest could be a likely candidate for the source of Roderick and Madeline's madness, it is not necessarily the only candidate. The Usher house itself seems permeated by evil. Roderick's inability, perhaps even his unwillingness, to leave it and start a new life suggests an individual pressured to conform to unhealthy tradition.
In Poe's "The Fall of the House of Usher," perhaps what the reader sees is the deterioration of Roderick and Madeline Usher's family, specifically its health: physical and mental. The cause of these goes back to tradition and conformity to tradition.
It would seem that the brother and sister may have had an incestuous relationship. Incest, among its social taboos, also causes birth defects, disease, and/or madness. If their parents had also practiced incest, this would have easily caused problems for their children. Madness (or evil) seems to be the central focus of the story as Roderick's behavior becomes more unbalanced and frightening as the story progresses, and even Madeline's health deteriorates.
In terms of tradition, especially in long-gone traditional houses of power, marriages of cousins would take place to keep the control within one family. This, however, often caused a high mortality rate among the offspring of these "couples," as well as madness.
In Madeline's case, she becomes so ill that her brother mistakenly buries her alive believing she is dead. When she reappears, having escaped from her entombment, madness swirls around the pair, the weather seems to have gone wild, and the unnamed narrator flees, only to soon witness the house's destruction when lightning bolts split the structure in two. The family name of the last two heirs of the Usher line is destroyed at the end as well.
Poe speaks to the nature and causes of evil. What has gone on in this house might have been considered by Poe to be evil--whether for the sake of evil itself or because of unnatural behaviors, we cannot be sure.