What cultures allowed incest, anyway? (Honest question; I don't know.) Did any of them not fall into the amoral warrior society mold? You haven't done much to prove it's not a matter of correlation more than anything. That's what I was getting at.
Eh. The easiest division of society (well, the part of society of the opposite gender) into "people I may furgle" and "people I may not furgle" relies on considering one set of cousins "honorary siblings" and the other as fair game, usually with completely different words for them. Cross-Cousin Marriage, it's called. (In Latin, for example, the early system got lost somewhere around the classical period, but before that they had what anthropologists call an Omaha Kinship System, after the native american language for which it was first analysed. It is indeed the most common classification of relatives around the world, although others exist.)
My father's daughter is my sister.
My father's brother's daughter is my sister.
My father's father's brother's son's daughter is my sister. etc. The same goes for my mother's sister's daughter - indeed, if the pattern is unbroken (but in reality, it frequently is), it's the same thing.
But my mother's brother's daughter is my future wife.