The editor above gives you a very accurate answer to your question.
To add, Aunt Alexandra has done something that few are ever able to: she moved Atticus to act in a way outside of his comfort zone. She manipulated her way onto the scene within the presence of a pretty powerful character. Maybe part of the purpose the author has Aunt Alexandra there is to be what we call a 'foil' in literature. A foil is an exact opposite to another character. Even though she is Atticus' sister, Aunt Alexandra is indeed an exact opposite of Atticus in many ways.
She wants the kids to think in terms of social appropriateness. Atticus wants them to think in terms of moral righteousness.
Hope that helps contribute to the ideas that the previous editor gave.
When Scout and Jem arrive home from Calpurnia's church at the end of Ch. 12, they find Aunt Alexandra sitting in a rocking chair on the porch. They ask her if she's come for a visit, but her reply is much more discouraging (at least for Scout).
"Well, your father and I decided it was time I came to stay with you for a while."
"For a while" in Maycomb meant anything from three days to thirty years. Jem and I exchanged glances.
"Jem's growing up now and you are too," she said to me. "We decided that it would be best for you to have some feminine influence. It won't be many years, Jean Louise, before you become interested in clothes and boys--"
I could have made several answers to this: Cal's a girl, it would be many years before I would be interested in boys, I would never be interested in clothes . . . but I kept quiet.
So, it seems that Atticus and Aunt Alexandra have decided that Scout needs "feminine influence". That is, she's going to teach Scout how to be a girl, in Aunt Alexandra's definition of the term. Of course, it's more likely that Aunt Alexandra decided Scout needed a woman's guidance, & Atticus couldn't say no. Of course, it's also telling that Alexandra doesn't consider Calpurnia a suitable female role model for Scout, most likely because she's black and in the position of a servant in the Finch household. Although all the Finches (Atticus included) consider Calpurnia as a second mother to Jem and Scout, Aunt Alexandra simply cannot accept this. She may also simply not trust the role to anyone but herself.