People often ask the question, “who is Cain’s wife?” A proper response to this question is to ask back “who was Seth’s wife?” The answer to both questions is relatively the same, they married their sisters. People often ask this question because they don’t actually know what the bible says about incest. The modern-day definition of incest is based on ideas about breeding offspring from parents that are genetically close, but in the ancient world siblings, half-siblings and cousins married all the time. How else would there be “royal bloodlines” in various cultures?

The Bible doesn’t limit incest to sex with blood relatives, in fact, a man is in sin if a man has sex with his non-blood-related stepmother (father’s wife or even ex-wife), because she is his father’s wife or ex-wife (Gen 35:22 & Gen 49:4, Lev 18:8, Lev 20:11, Deut 22:30, & 1 Cor 5:1). This indicates that those rules about incest are not explicitly about genetics but about relationships. In the Genesis era, it only would have been a sin to have a vertical sexual relationship (parents/grandparents having sex with their children/grandchildren). For example, Lot’s daughters get him drunk and have sex with him after escaping Sodom to the wilderness, because they are afraid they are not going to find husbands. However, horizontal relationships (siblings, half-siblings, and cousins), and diagonal relationships (aunts/nieces and uncles/nephews) are not explicitly labeled as sin and are done themselves by the pre-Moses patriarchs. Abraham and Sarah are half-siblings with the same father (Gen 20:12), Abraham’s brother, Nahor marries his other brother Haran’s daughter Milcah, making them uncle and niece (Gen 11:29), and Moses’ own parents (Amram and Jochebed) are nephew and aunt (Ex 6:20). Although Leviticus doesn’t ban niece-to-uncle marriages so Nahor isn’t a conflict with the Mosiac Law.

We don’t see tight restrictions on these things in the bible until the time of Moses. These are God’s terms and conditions for the Sinaitic covenant mediated by Moses. The Sinaitic covenant is primarily for the nation of Israel, though it does include moral laws that were sin to break before Moses like murder, adultery, and others, which apply to all nations. In Leviticus is where we see more marriage restrictions on both blood and non-blood relatives like siblings & half-siblings (Lev 18:9, Lev 20:17), step-siblings (Lev 18:11), and aunts (Lev 18:12-13, Lev 20:19). Uncles to niece and cousin relations are allowed however aunt to nephew relations are off-limits even though they should categorically be the same. This could be because of the male/female hierarchy as well as the limits on polygamous marriages to women that are related to each other. A grandfather giving his daughter away to his grandson would be improper because it is crossing genealogy lines. Males give away female relatives in marriage when their fathers are deceased, so it would be improper for a man to give his sister away in marriage to his son. For example, Ishmael had been dead for 14 years when Esau married his daughter Mahalath in Gen 28:8-9, so she was given away by Nebaioth the firstborn son of Ishmael who is mentioned in verse 9. Marrying an aunt on the mother’s side may be more about their sisterly relationship being disrupted by them becoming a mother and daughter-in-law wife pair, even though they are sisters. This could be connected to the ban on polygamous marriages to sister pairs and mother-daughter pairs. Essentially the rule was that women could marry men in the same or a higher generation, but not lower, and men could marry women in the same or lower generation but not higher. Moses’ own parents violate these laws, because his mother is his father’s aunt, yet it doesn’t count against them because it was not given yet. These are just some of the most obvious verses about sex with relatives. These sex practices along with others were even banned in some other world cultures as well (1 Cor 5:1). God warns the Israelites that the Canaanites are doing these things, and he doesn’t want them to copy these same habits when they get to the land.