It's not exactly clear why Aunt Alexandra suddenly decides to come and live with Atticus and his family in Maycomb. She leaves her husband behind at Finch's Landing, and tells Scout that she plans "to stay with you for a while."
"For a while" in Maycomb meant anything from three days to thirty years. (Chapter 13)
She comes to stay for several reasons. With the upcoming trial of Tom Robinson, Atticus will be busier than usual and away from home for longer hours. Alexandra also believes the children--and Scout in particular--need a woman's touch around the house. Atticus never admits this completely, and he beats around the bush trying to explain Alexandra's presence to his kids.
"We felt it was time you children needed--well, it's like this, Scout... Your aunt's doing me a favor as well as you all. I can't stay here all day with you, and the summer's going to be a hot one." (Chapter 13)
Alexandra is Atticus's older sister, and she is used to getting her way. She dominates her lazy husband, Jimmy, who spends most of his day fishing. She tries to dominate Atticus as well until he puts his foot down when Alexandra suggests he fire Calpurnia. Her aunt reminds Scout of Mount Everest: "... she was cold and there." She resembles her younger brother, Jack, and is a terrific cook: "Her cooking made up for everything." She hates Scout's "attire," believing she will never become ladylike until she starts wearing dresses regularly. Alexandra plays favorites, showering excess attention on her grandson, Francis, and taking his side after Scout "split my knuckle to the bone on his front teeth" at Christmas. After arriving in Maycomb, Alexandra is particularly "irritable" on Sundays due to her "corset."
She was not fat but solid, and she chose protective garments that drew up her bosom to giddy heights, pinched in her waist, flared out her rear, and managed to suggest that Aunt Alexandra's was once an hour-glass figure. From any angle, she was formidable. (Chapter 13)
(Alexandra's stay proves to be a fairly long one, since she is still a member of the household at the end of the novel.)
Aunt Alexandra's arrival in Harper Lee's To Kill a Mockingbird is more pertinent in literary terms and is used as a device by Lee that fascinatingly allows for Scout's coming of age in many ways. She is the anti-Atticus inasmuch as to help solidify Atticus's influence on Scout's life, and to extend and fully realize the mockingbird metaphor in the novel. Her staunch stentorian ways are completely at odds with Atticus's parenting, which though very much in keeping with parental discipline, is also about grounding Jem and Scout in their own individuality. Her style of parenting is more imposing than Atticus's, and that's how the reader sees it, as the story is in Scout's POV.
That she is a secondary character is made clear by the fact that we are not given much detail about the reasons of her moving in with the Finches, as also Scout's resistance to Aunt Alexandra's attempts to make Scout more ladylike in her attire and speech.