Just the fact that Aunt Alexandra comes to live with her brother adds to the contrast between them because the two siblings are completely different people. Aunt Alexandra is a snobbish, prejudiced person while Atticus is relaxed and not prejudiced. In chapter 13, Aunt Alexandra starts preaching her beliefs about the Finch family being above everyone else. She tells the family that every other family in the county has a "streak," just to show that Finches are better than everyone else. Everything is based on heredity with Aunt Alexandra, which gets old to listen to.
"Everybody in Maycomb, it seemed, had a Streak: a Drinking Streak, a Gambling Streak, a Mean Streak, a Funny Streak" (129).
Atticus kindly throws her "streaks" back in her face by suggesting that maybe the Finches have an incestuous streak because they are the first generation who didn't marry their cousins. Aunty is proud of her heritage and simply says that small hands are the only inheritance they received from "gentle breeding."
The situation in the Finch household escalates when Aunt Alexandra puts Atticus up to discussing heritage and the birds and the bees with the children. It doesn't work. It only makes things worse for Atticus because Scout starts crying. She cries because she feels the wind changing in her house and wants to know if they will have to live under Aunt Alexandra's domineering rule forever. Atticus sees that Aunt Alexandra's pushing and manipulating is disturbing the household's status quo in a negative way. Jem and Scout ask their father if he really wants them to remember everything Aunt Alexandra tells them. Atticus gives up and says, "I don't want you to remember it. Forget it" (134).
For the most part, then, the elements that bring a heightened sense of contrast to the Finch home are Aunt Alexandra's "preoccupation with heredity" and her manipulation of Atticus to support her ideas and teach the children what she believes. Fortunately, Atticus drops the effort, but unfortunately, Aunt Alexandra does not.
The contrasts are extreme. Scout thinks her aunt finds her dull while she knows Atticus finds her interesting. Alexandra has "river boat boarding school manners," and while Atticus is a gentleman, he is also very relaxed and unpretentious. She feels she is better than others, while Atticus is quick to tell her that their "generation's practically the first in the Finch Family not to marry its cousins," noting they are as "common" as anyone else. Alexandra wants Scout to behave like a lady, and while Atticus tries to reinforce his sister's wishes, Scout knows that he loves her the way she is, which, at the time, is more of a tomboy than a lady.