The reader never knows for sure exactly why Aunt Alexandra decided to pack her bags and leave Finch's Landing to live with Atticus in Maycomb. Alexandra believed that a "feminine influence" was needed in the household in order to transform Scout into a little lady, and she must have also realized that Atticus would be busier than ever with the trial and his legislative responsibilities, and that the children needed more adult supervision while he was away. It could be that Alexandra was also bored at the landing, and she seemed happy to get away from her husband, Jimmy.
Alexandra has the luxury of seeing Maycomb from the eye of an outsider, not that she has any problem immediately immersing herself in Maycomb society. She worries about Atticus and the repercussions the trial may have upon the family. Her attempts to teach the children about "fine folks" and "gentle breeding" meet with mixed results, and when Atticus puts his foot down about his sister's suggestion to fire Calpurnia, Alexandra recognizes that Atticus has more control over his household than she imagined. Alexandra may never entirely agree with her brother's actions and beliefs, but she supports and worries about him.