The narrator learns that that the Usher family tree is unusual in that only one child every generation survives to have more children. This suggests that the family is not very healthy, as we can see in the case of Roderick and Madeline. It also means tht Roderick and Madeline have no aunts or uncles or cousins. Their father had no brothers or sisters who survived.
The implication, too, is that first cousins (the "trifling variation" in the family line), and even brothers and sisters, had had children by each other, which would lead to there being no branches. First cousin marriage was not unusual among important families in nineteenth-century Europe. Brother and sister incest would be illegal, but that doesn't mean it didn't happen, Incest would, however, lead to a physical weakening of the family line, which is what we see in Roderick and Madeline, who are both frail. Madeline, in fact, is terminally ill.
Roderick tells his friend that Madeline's death
would leave him (him the hopeless and the frail) the last of the ancient race of the Ushers.
There is no thought, apparently, of Roderick marrying a woman from the outside world to continue the family line.