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Atticus's voice was even: "Alexandra, Calpurnia's not leaving this house until she wants to. You may think otherwise, but I couldn't have got along without her all these years. She's a faithful member of this family and you'll simply have to accept things the way they are." (14.28)
Aunt Alexandra's idea of family is exclusive (kick out those who aren't worthy of being counted in), while Atticus's is inclusive. She arranges family by blood, while Atticus considers affection and loyalty. Basically, it boils down to: do you judge a man (or woman) by his birth—or by his life?