Aunt Alexandra is significantly different than her brother Atticus in many ways. One of them is precisely her treatment of others who are different than herself.
Aunt Alexandra is kind of stuck in her ways as opposed to Atticus, who sees all men as worthy of all the compassion and justice as can be possible.
In the case of the Cunninghams, the reality was that they were very poor and had very little to go on with. Atticus, however, would express his appreciation for Walter Cunningham for trying to get by within his circumstances.
Aunt Alexandra, however, was very aware of social differences and did not let Scout or Jem play with the little Cunningham boy. In fact, she said he was "trash" and considered him dirty, yappy, and uneducated.
In all, Alexandra is the complete opposite of Atticus when it comes to treating people and feeling compassion for their needs.