three groups that we know of required brother-sister marriages for their high nobility: the ancient Egyptians, the Incas of Peru, and the old kingdom of Hawaii (Beals and Hoijer 1965). Some groups also allow sex between fathers and daughters. The Thonga, a tribe in East Africa, permit a hunter to have sexual intercourse with his daughter before he goes on a lion hunt. And a tribe in Central Africa, the Azande, permit high nobles to marry their own daughters (LaBarre 1960).
http://wps.ablongman.com/ab_henslin_essentials_5/12/3197/818642.cw/index.html
Still doesn't make it right though.