Aunt Alexandra believes that some families are better than others.
Aunt Alexandra feels that Atticus has not instilled in his children how important their family name is in Maycomb. Notice that Scout does begin the novel by describing the Finch family.
Scout sums up Aunt Alexandra’s position on family breeding.
I never understood her preoccupation with heredity. Somewhere, I had received the impression that Fine Folks were people who did the best they could with the sense they had, but Aunt Alexandra was of the opinion, obliquely expressed, that the longer a family had been squatting on one patch of land the finer it was. (ch 13)
Atticus tells Jem and Scout that Aunt Alexandra expected him to teach them that they were not “run of the mill people” but rather the product of “several generations of gentle breeding” (ch 13). Clearly, Aunt Alexandra did not think that Atticus could handle all that and the trial when the Finch family would be in the spotlight. Atticus tells them:
“She asked me to tell you you must try to behave like the little lady and gentleman that you are. She wants to talk to you about the family and what it’s meant to Maycomb County through the years, so you’ll have some idea of who you are, so you might be moved to behave accordingly.” (ch 13)
Atticus is extremely uncomfortable telling them this. It was not uncommon for families with land, who had been in the same place with the same money for generations, to consider themselves better than other families. Scout thinks that people are better if they act better, but Aunt Alexandra’s view was the more common, adult one. Scout looks at the issue not just as a child would, but as Atticus would. It seems he has taught her well after all.