Book 10 describes the family as the place where one person’s joy can become everyone’s joy. When the father is pleased, the whole family is pleased; when the mother is pleased, the whole family is pleased; even a child’s delight can become shared joy.
That means family is not only where duties are assigned. It is the smallest social form where flourishing stops being zero-sum and becomes simultaneous.
This is one reason the chapter treats family as a preview of heaven. Heaven is not merely moral order. It is a world where another person’s good does not threaten my own.