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.

So obviously, neither Finch child is really looking forward to having their aunt stay for any length of time. Then she reveals her true purpose:

"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--"

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. Knowing Aunt Alexandra's personality, it's more likely that she decided Scout needed a woman's guidance, & Atticus couldn't say no. The fact that Alexandra doesn't consider Calpurnia a suitable female role model for Scout also reveals much about her character. With Alexandra's emphasis on status and family history, she has no faith in Calpurnia, 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. So, she makes herself a place in their lives, for better or worse.