What's weird about the Usher family tree?

Poe's "The Fall of the House of Usher" recounts the story of its narrator's visit to the secluded estate of his boyhood friend Roderick Usher.  While describing his initial knowledge of the estate, the narrator states:

I had learned, too, the very remarkable fact, that the stem of the Usher race, all time-honoured as it was, had put forth, at no period, any enduring branch; in other words, that the entire family lay in the...

Poe's "The Fall of the House of Usher" recounts the story of its narrator's visit to the secluded estate of his boyhood friend Roderick Usher.  While describing his initial knowledge of the estate, the narrator states:

I had learned, too, the very remarkable fact, that the stem of the Usher race, all time-honoured as it was, had put forth, at no period, any enduring branch; in other words, that the entire family lay in the direct line of descent, and had always, with very trifling and very temporary variation, so lain.

In the passage above, the narrator indicates that what is unusual about the "Usher race" is that their family tree does not proliferate into a multitude of family lines, "any enduring branch." Instead, Poe's narrator emphasizes that the Usher family tree is a straight line, a "direct line of decent" with little to no "variation."

The implication is that the Ushers--not unlike the European royalty of prior centuries--are the product of generations of incest.  The narrator buttresses this reading in his comparison of Roderick and his sister throughout the text, casting Madeline as the Gothic double of the more prominent Roderick.