The structure of an average American's family tree is pretty simple. A mom and a dad, their parents and parents of their parents, their kids and then spouses of kids and their parents. Nobody was initially related to each other and unless it is a married couple who came from two different families, there are no sexual relationships whatsoever. In the Greek tragedy "Oedipus the King" , Sophocles wrote about the ruler of Thebes who crossed the line of relations and married his mom. That changed his family structure and it made it disgusting not only for readers, but also for his family.
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