The major way that the author uses foreshadowing while explaining the reason for Aunt Alexandra's stay really lies in Aunt Alexandra's sense of family pride. She says she has come because Scout was growing up and maybe needed a woman's guidance. Then there follows a description of how well she fit into Maycomb and its society. A little later she talks to Atticus about Scout's behavior telling him that Scout needs to understand her family history so perhaps she will curb her behavior accordingly. Atticus tries to talk to her, but cannot follow through because he does not really believe any of this family pride nonsense. All of this of course, foreshadows the coming trial in which her father will behave in a way that angers the society of Maycomb and isolates him and his family from that society. Atticus breaks from Maycomb society in a much more serious way than Scout's childish behavior could ever do. In a smaller way, Atticus foreshadows the time the trial will take from him when he tells Scout that Aunt Alexander is doing him a favor by coming because he cannot be around all day.
From the second she enters the home of Atticus, Jem and Scout, Aunt Alexandra's presence lends a sense of foreboding to Harper Lee's novel To Kill a Mockingbird. Lee begins Chapter 13 with the following:
“'Put my bag in the front bedroom, Calpurnia,' was the first thing Aunt Alexandra said. 'Jean Louise, stop scratching your head,' was the second thing she said."
With this introduction, Lee is clearly positioning the reader for an unwelcome intrusion into the lives of Atticus, his children, and their housekeeper, Calpurnia. Right off the bat, Aunt Alexandra is depicted as autocratic, demanding of others and disrespectful of the manner in which Atticus has run his home in the past. Initially, Alexandra's entrance does not necessarily augur ill for Scout, Lee's young narrator. The outlook, however, turns decidedly bleak when Alexandra informs her niece that this is not a short visit. Rather, Alexandra informs the children, it is a long-term visit that marks a sudden and—for Scout, at least—unpleasant development.
Earlier in Lee's novel, in Chapter 9, Scout discusses Aunt Alexandra in the context of the latter's anticipated Christmas visit to Maycomb. In that chapter, Scout makes it very clear that her aunt is, at least as far as this precocious six-year-old is concerned, persona non grata:
"Had I ever harbored the mystical notions about mountains that seem to obsess lawyers and judges, Aunt Alexandra would have been analogous to Mount Everest: throughout my early life, she was cold and there."
When, in Chapter 13, then, Aunt Alexandra announces that she has come to stay, and that her arrival is due in no small part to her—and, to a lesser extent, Atticus'—concerns about the manner in which Scout is being raised, that aforementioned sense of foreboding envelopes the story. Whereas the main conflict in the Finch home heretofore is the approaching rape trial of Tom Robinson, Alexandra's presence will now introduce an entirely new problem for Scout beyond her father's defense of a black man accused of raping a white woman. Not only is Alexandra judgmental and disapproving of her brother's decision to defend Tom Robinson, but she is equally disapproving of Scout's friendship with Walter Cunningham and with Scout's tendency towards what were considered more masculine activities. Note, for instance, the manner in which she addresses Scout in the passage above: "Jean Louise," rather than the young girl's preferred nickname.
Aunt Alexandra's arrival casts a dark cloud over the Finch household. We already know from Chapter 9 that Scout dislikes her aunt and that Alexandra is a generally unpleasant individual. Her entry into the Finch home and immediate criticism of Scout for scratching her head simply do not bode well for Scout.