I'm pretty sure that on the grass-root level, incest is viewed as a thing to be avoided. (In autocratic societies, with the autocrats spreading their genes among their subjects, this is less of a problem - except among the autocrats.)
Sibling or offspring marriages are recorded e.g. for Elamle-Ata (Daughter-Wife of her father, and sister-mate to Miirdek as well IIRC.). IMO this happens where a divine or heroic bloodline is desperately kept as pure as possible, since it is a magical requirement for the welfare of that society.
Cultures that have Sacred Marriages of leaders are luckier - even when priest(ess)hood is sort of hereditary (Feathered Horse Queens, Shaker Temple priestesses), the heroic partners change.
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