Different family dynamics are largely explored through contrast here: rich families and poor ones, families with numerous kids or very few, families lacking moms or dads, families living in villages or in fancy abbeys. Family identity is extremely important to all of the characters and helps to define their beliefs and their behavior. Family is a crucial component of individual identity and it is a largely inescapable factor in how characters are perceived by others in this text. Northanger Abbey focuses mainly on the Morlands, the Thorpes, and the Tilneys, and also explores how different types of families function in society and how they interact with each other.
The three sibling pairs featured prominently in the text actually have more similarities, in terms of their interactions and behavior, than they do differences.
One of the major lessons of the book is that you shouldn't judge individuals by their families.