Leviticus 18:9 and 20:17 deliver absolute capital prohibitions against marrying or having relations with a sibling or half-sister, whereas Genesis 20:12 features Abraham explicitly married to his half-sister Sarah without explicit condemnation. Traditional harmonization explains that these early primeval unions were necessary for population growth and occurred long before the formal mosaic code was delivered, while critical scholarship views it as an evolutionary development in ancient kinship taboos.