Brother-sister relationships appear in Egyptian, Norse, and Greek legends, often between heroic deities, and in many of those tales the offspring from those pairings are destined for something equally mythic.
Life with little children is learning how to do everything with one hand while carrying a baby in another.
Kenneally, related to a prominent Fianna Fáil politician, admitted his abuse.
Paraphrasing Lévi-Strauss's argument, without this avoidance, the rivalries for power between age-sets, coupled with the close bonds of sharing between age-mates, could lead to a sharing of daughters as spouses.