People often ask the question, “Who is Cain’s wife?” A proper response to this question is to ask in return, “Who was Seth’s wife?” The answer to both questions is the same: they married their sisters. People often ask this question because they don’t know what the Bible says about incest. The modern-day definition of incest is based on ideas about breeding offspring from genetically close parents, 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 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 would only 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 committed by the pre-Moses patriarchs themselves. 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 an uncle and niece marriage (Gen 11:29), and Moses’ own parents (Amram and Jochebed) are nephew and aunt (Ex 6:20). Theres relationships are later prohibited in the Law of Moses. Although Leviticus doesn’t ban niece-to-uncle marriages, so Nahor isn’t in conflict with the Mosaic Law.

The Law of Moses is 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 on prohibiting behaviors 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 with nephews (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 who are related to each other. A grandfather giving his daughter away to his grandson would be improper because it crosses genealogical 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 prohibited at the time. 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.