Aunt Alexandra shows up at the end of Chapter 12 of To Kill a Mockingbird. Supposedly, she is there to give some feminine guidance, be a mother figure, for Scout. It is clear to the reader, however, that Calpurnia has been filling that role quite well prior to Alexandra's arrival. Alexandra claims that both she and Atticus decided it was time for Scout to have a feminine influence. But Atticus indicates that it was more Alexandra's idea than his.
Near the end of Chapter 13, Atticus reveals why Alexandra really has come to stay with the Finches.
“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” he concluded at a gallop.
Atticus spoke these words to the children "at a gallop" because he wanted to get them out. Perhaps Alexandra was listening. Atticus did not believe in any of this. He wanted his children to behave and become upstanding citizens. But he didn't want them to think they are better than others simply because of family or social traditions. At the end of the chapter, Atticus tells the children to forget what he just said.
Alexandra feels inclined to impress upon the children their family heritage. Atticus agrees to this, perhaps thinking he could use some parental help during the trial. Or, perhaps Atticus recognizes that Alexandra needs something to do. In Chapter 9, Scout notes that Alexandra's son, Henry, left home "as soon as was humanly possible." Although Atticus disagrees with Alexandra's views on social traditions and "knowing your role," he recognizes that most people in Maycomb think the way she does. Atticus will continue to reject this insistence on social traditions but he is willing to have Alexandra (who does have good intentions) stay with them to give her a sense that she has some purpose.