Through the missionary circle we learn that Aunt Alexandra does support her brother and is not as racist as we thought.
In chapter 9, we learn some unsavory things about Aunt Alexandra and she does not seem like a good person. She does not take the time to understand or appreciate Scout. The worst part is what Fraces overheard her say.
Grandma says it's bad enough he lets you all run wild, but now he's turned out a nigger-lover we'll never be able to walk the streets of Maycomb agin. He's ruinin' the family, that's what he's doin'." (ch 9)
This seems to indicate that she does not support her brother, and considers him a disgrace to the family for defending Tom Robinson. It also establishes her as horribly racist. Later we learn she is classist too, when she calls Walter Cunningham “trash” in chapter 13.
The trial changes things. When Alexandra calls Atticus “Brother” and says she is sorry, we see her beginning to come around. She may still have prejudices, but they are beginning to be buried.
Scout has come to understand Aunt Alexandra a little better by the time we see the ladies missionary circle in action. She has a low opinion of the ladies and the proceedings.
Today Aunt Alexandra and her missionary circle were fighting the good fight all over the house. (ch 24)
The important incident is between Mrs. Merriweatherand Miss Maudie. Mrs. Merriweather basically calls Atticus “misguided” in his own house, and Miss Maudie intervenes, asking if the cook’s food sticks going down. Scout notices an exchange between Alexandra and Maudie that shocks her.
She gave Miss Maudie a look of pure gratitude, and I wondered at the world of women. Miss Maudie and Aunt Alexandra had never been especially close, and here was Aunty silently thanking her for something. (ch 24)
Miss Maudie stood up for Atticus because she is never afraid to speak her mind and point of hypocrisy where she sees it. Alexandra was grateful, because she really did care about her brother and did not have the guts to stick up for him herself. Scout and the reader get a new perception of Aunt Alexandra after this, seeing her softer side.