How does Lee use humor to show that she does not approve of using family heritage as a way to judge people? How is this significant to the character of Aunt Alexandria?

Aunt Alexandra goes on and on with explanations of the different family streaks; the Merriweather family has a morbid streak, and the Penfields have a giddiness streak. Other families have drinking streaks, mean streaks, and gambling streaks. Atticus responds to her rant by teasingly reminding her of the Finch's Incestuous Streak. She doesn't get the humor (or simply can't see he's teasing her), and she responds that the Finch streak is responsible for the family's inherent trait of small hands and feet. This conversation is significant to the character of Aunt Alexandra because it exemplifies her feeling of family pride.