I think the entire Finch household affects Aunt Alexandra. When the reader is first introduced to her, the reader is meant to have a certain amount of disdain for Aunt Alexandra. Part of that is because the story is told through Scout's eyes and partly because Aunt Alexandra is such a departure from the Finch household.
Aunt Alexandra is focused on the importance of family name. The Finch name is a good name with a good reputation and she believes that Atticus isn't doing much to make sure it stays that way. She disagrees with Atticus taking the Robinson case, she doesn't like how much of a tomboy Scout is, and she doesn't like how the Finches treat Calpurnia like a member of the family. Aunt Alexandra is all about appearances and propriety.
As the novel progresses though, the reader sees a general softening of her character. This is evident late in the novel when Scout learns about Tom's attempted escape and death. She is also much more sympathetic to Atticus and his actions around the town. I don't believe it's any one person who causes a great change in Aunt Alexandra, rather, I think it's a slow transformative process worked on by the entire Finch family.