It is hard to believe that brothers and sisters could be so different. Scout had her own hypothesis about Alexandra.
... when Jem told me about changelings and siblings, I decided that she had been swapped at birth, that my grandparents had perhaps received a Crawford instead of a Finch.
Despite Alexandra's high-handed opinion about her family upbringing and the need for her to provide motherly guidance for Scout, she has a poor track record. Her son, Henry, "left home as soon as was humanly possible, married and produced Francis." Aside from Bob Ewell (and his chip off the block, Burris), Francis is the most detestable character in the novel, and Alexandra apparently pampers him during his visits to Finch's Landing.
As for the Cunninghams, they would probably be considered "poor white trash" in many areas of the South. However, the Ewells have a stranglehold on this distinction in Maycomb, so the Cunninghams seem like "plain folks" in comparison. However, the Cunninghams are moonshiners who attempted to lynch Tom Robinson, so they are not among Maycomb's finest citizens--just not in the same class as Bob Ewell.