Like so many of British descent, Aunt Alexandra is extremely class conscious. Shortly after her arrival prompted by her desire to lend familial support to her brother and re-establish the dignity of the Finch name, which has been maligned by the aspersions as those such as Mrs. DuBois. Shortly after her arrival Aunt Alexandra informs the children of the "caste system" in Maycomb, as Scout refers to it.
Later, in Chapter 23, in a discussion with Jem about Tom Robinson's trial, Atticus praises a Cunningham for having enough doubt to hold up the jury's verdict for an hour, adding that if there had been two, there would have been a hung jury. After hearing this, Scout decides that she will befriend Walter Cunningham once school resumes; however, Aunt Alexandra interrupts with "We'll see about that." When Scout asks her, "Why not, Aunty? They're good folks," Alexandra declares,
"Jean Louise, there is no doubt in my mind that they're good folks. But they're not our kind of folks....you can scrub Walter Cunningham till he shines, you can put him in shoes and a new suit, but he'll never be like Jem. Besides, there a drinking streak in that family a mile wide. Finch women aren't interested in that sort of people."
The Cunninghams, according to Aunt Alexandra, are lower class people, not of the ilk of the Finches. Also, they have alcoholism in their lineage.