Aunt Alexandra has this strange notion that the characteristics of each family are handed down from generation to generation. So if a family has the reputation of being bad—like the Ewells, for example—it's because they've always been bad, and, what's more, always will be bad. Such families have a genetic bad streak that simply cannot be erased by the passage of time.
By the same token, "good" families like the Finches are the product of good breeding, which stretches back several generations. According to Aunt Alexandra, the Finches are a good family because they have a good streak in them. Because of this happy genetic inheritance, they have always been a good family and always will be a good family, come what may. It's all in the genes.
An additional factor in making a family "good" is the amount of time they've been occupying a particular plot of land. On Aunt Alexandra's view, the ownership of land, especially over a long period of time, confers power, status, and respectability, all the things that really matter in life to this crashing snob.