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Some anthropologist, maybe Margaret Mead, was investigating a so-called primitive tribe and was asking about whether they allowed brother/sister marriages. The tribesmen thought it was a ridiculous question--such a man would have no brothers-in-law.

I once heard a talk about brother-sister marriage in Egypt, between ordinary people (not pharaohs and their relatives). I think it must have been from the Roman period. Apparently it happened and was not as detrimental in the long run as might be expected--those who inherited bad genes from both sides died off and those genes were eliminated from the gene pool.

Uncle/niece marriages were acceptable in Sparta--Leonidas was married to his brother's daughter. At Rome when the emperor Claudius married his own niece, Agrippina the Younger, people were appalled. But Livy has a case of a man married to his own niece in the 5th century BC and it is treated as nothing out of the ordinary.