Aunt Alexandra decided to come to stay with Atticus and his family for a while because she wanted to provide to Scout some "feminine influence" to her, how to become a proper woman and not be a tomboyish sort of person- which is so un-ladylike. She is trying to make Scout to become a more refined and dignified person, unlike her uncivilized type she was in the past, when she only hang out with boys. She should broaden up her social circle and know more different types of people, maybe in the process teaching her some proper-lady behavior and manner
Aunt Alexandra says that she moves into provide Scout with a women's influence, but the timing of her move indicates that there may be other factors involved. She moves in just before the start of the Tom Robinson trial, and although she disapproves of her brother defending Robinson, as family this may be her way of showing her brother support. She knows that Atticus will be very preoccupied with the trial and will have little time for his children and this is one way she can help him without being involved with the trial Secondly, her quick and complete involvement in town life suggest that part of the reason for the move is her own need of the social outlets that life in town has to offer compared to the isolation at Finch's Landing. It surely seems that her coming to town has more implications than her desire to civilize Scout.