The Grammar of Kinship: God's Law on Incest Text: Leviticus 18:6-18
Introduction: Walls and Fences
We live in a generation that has taken a sledgehammer to every load bearing wall in the house, and is then bewildered when the roof collapses. Our culture has declared war on all boundaries, all distinctions, and all definitions, particularly in the realm of human sexuality. The world tells us that love is love, that desire is its own justification, and that any restriction on sexual expression is an act of oppressive bigotry. But what they call freedom is, in reality, a headlong plunge into chaos, confusion, and utter degradation. They have sown the wind of sexual autonomy, and they are reaping the whirlwind of societal collapse.
Into this chaos, the Word of God speaks with stark, architectural clarity. The laws we are about to consider in Leviticus 18 are not arbitrary restrictions designed to make life miserable. They are not suggestions. They are divine fences. They are walls that God Himself erects to protect the most basic and fundamental institution of human society: the family. These laws define the grammar of kinship. They teach us what a family is, how it is structured, and what relationships are appropriate within it. To violate these laws is not simply to break a rule; it is to speak familial gibberish. It is to undermine the very foundation upon which healthy individuals and a healthy society are built.
These prohibitions against incest are a direct polemic against the practices of the Egyptians, from whom Israel had just come, and the Canaanites, into whose land they were going. Those cultures were saturated with sexual perversion, including the very acts forbidden here. God is setting His people apart. He is teaching them that holiness is not an abstract concept; it has to do with whom you sleep, how you build your house, and how you order your loves. We are not to be like the nations. And as we will see, these laws are not dusty relics of a bygone era. They are rooted in the created order itself, and their general equity, their underlying principle, is as binding today as it was the day Moses wrote them down. To ignore them is to invite the judgment of God, not just on individuals, but on the land itself.
The Text
‘None of you shall approach any blood relative of his to uncover nakedness; I am Yahweh. You shall not uncover the nakedness of your father, that is, the nakedness of your mother. She is your mother; you are not to uncover her nakedness. You shall not uncover the nakedness of your father’s wife; it is your father’s nakedness. The nakedness of your sister, either your father’s daughter or your mother’s daughter, whether born at home or born outside, their nakedness you shall not uncover. The nakedness of your son’s daughter or your daughter’s daughter, their nakedness you shall not uncover; for their nakedness is yours. The nakedness of your father’s wife’s daughter, born to your father, she is your sister; you shall not uncover her nakedness. You shall not uncover the nakedness of your father’s sister; she is your father’s blood relative. You shall not uncover the nakedness of your mother’s sister, for she is your mother’s blood relative. You shall not uncover the nakedness of your father’s brother; you shall not approach his wife; she is your aunt. You shall not uncover the nakedness of your daughter-in-law; she is your son’s wife; you shall not uncover her nakedness. You shall not uncover the nakedness of your brother’s wife; it is your brother’s nakedness. You shall not uncover the nakedness of a woman and of her daughter, nor shall you take her son’s daughter or her daughter’s daughter, to uncover her nakedness; they are blood relatives. It is lewdness. And you shall not marry a woman in addition to her sister as a rival while she is alive, to uncover her nakedness.
(Leviticus 18:6-18 LSB)
The Foundational Principle (v. 6)
The chapter lays down the central command before listing the specific applications.
"‘None of you shall approach any blood relative of his to uncover nakedness; I am Yahweh." (Leviticus 18:6)
The prohibition is broad: do not approach a "blood relative." The Hebrew is literally "flesh of his flesh." This is a direct echo of the language of marriage in Genesis 2, where a man and wife become "one flesh." God is teaching us that there are two kinds of "one flesh" unions. The first is the sexual union in marriage, which creates a new family. The second is the union of kinship, which is established by birth. The central point of this chapter is that you must not confuse the two. You must not try to create a marital union where a blood union already exists.
The phrase "to uncover nakedness" is a euphemism for sexual intercourse. It is not about accidental exposure. It is about the intimate, covenantal act of sexual union. And notice the reason given: "I am Yahweh." This is not a pragmatic argument based on genetics or social stability, though those things are certainly protected by these laws. The ultimate reason is theological. God is the one who defines reality. He is the one who designed the family, and He gets to set the rules. To violate these boundaries is to rebel against Him personally. It is an act of cosmic treason against the Creator's design.
Protecting the Core Family (v. 7-11)
The laws then move from the general to the specific, starting with the most immediate family relationships.
"You shall not uncover the nakedness of your father, that is, the nakedness of your mother... You shall not uncover the nakedness of your father’s wife; it is your father’s nakedness." (Leviticus 18:7-8)
The first prohibition is against sleeping with one's mother. But notice how it is phrased. It is an offense against the father. To sleep with your mother is to "uncover your father's nakedness." Why? Because in marriage, the two become one flesh. A man's wife is, in a covenantal sense, him. This is why the Apostle Paul can say that he who loves his wife loves himself. To violate a man's wife is to violate him in a profound way. This law protects the sanctity of the parental unit, which is the foundation of the family. It also protects the authority of the father. For a son to do this is a grotesque act of rebellion, like that of Reuben or Absalom, a direct assault on the patriarchal order.
Verse 8 extends this to a stepmother ("your father's wife"). Even if she is not your biological mother, she has entered into a one-flesh union with your father. Her nakedness is therefore his nakedness. This shows that the prohibitions are not merely about bloodlines, but about covenantal relationships. The family is defined by more than just DNA; it is defined by vows and covenant headship.
"The nakedness of your sister... their nakedness you shall not uncover." (Leviticus 18:9)
The prohibition against incest with a sister, whether a full sister or a half-sister, protects the sibling relationship. Brothers and sisters are to have a relationship of purity, protection, and platonic love. Introducing a sexual element destroys this. It confuses the categories. A brother is meant to be a protector of his sister's purity, not a predator upon it. This law builds a wall around that sacred duty.
Generational Boundaries (v. 10, 12-17)
The law continues to draw lines, moving outward to aunts, grandchildren, and in-laws, consistently reinforcing the structure of the extended family.
"The nakedness of your son’s daughter or your daughter’s daughter, their nakedness you shall not uncover; for their nakedness is yours." (Leviticus 18:10)
A man is forbidden from relations with his granddaughter. The reason given is striking: "for their nakedness is yours." Just as a wife's nakedness belongs to her husband, a child's nakedness, in this protective sense, belongs to the family head. A grandfather's role is one of patriarchal blessing and protection, not sexual exploitation. To violate a granddaughter is to violate oneself, to commit a kind of spiritual self-mutilation. It is a confusion of generations. You cannot be both father and husband to the same line.
"You shall not uncover the nakedness of your father’s sister... your mother’s sister... your father’s brother... she is your aunt." (Leviticus 18:12-14)
The prohibitions concerning aunts and uncles by marriage reinforce the same principle. These individuals are part of the parental generation. They are to be honored as such. To have a sexual relationship with an aunt is to confuse the generational lines. It is to treat someone who stands in the place of a mother as a potential wife. Again, this is speaking familial nonsense. God's law demands clarity in our relationships.
The same logic applies to a daughter-in-law (v. 15) and a brother's wife (v. 16). The daughter-in-law is one flesh with your son. To violate her is to violate your son. The brother's wife is one flesh with your brother. To violate her is to violate your brother. This was the sin of Herod, which John the Baptist rebuked him for, and for which he lost his head. John the Baptist died for the enduring validity of Leviticus 18:16. These are not trivial matters.
Compound Prohibitions (v. 17-18)
The final verses address more complex, but equally destructive, forms of confusion.
"You shall not uncover the nakedness of a woman and of her daughter... It is lewdness." (Leviticus 18:17)
Here, a man is forbidden from marrying a woman and also her daughter or granddaughter. This creates an impossible situation. A man would be both husband and father-in-law to one person, and husband and grandfather-in-law to another. The relational lines become a tangled, incomprehensible mess. God calls this what it is: "lewdness." It is a wicked confusion of the created order.
"And you shall not marry a woman in addition to her sister as a rival while she is alive, to uncover her nakedness." (Leviticus 18:18)
This verse has been the subject of much debate, but its principle is clear. A man is not to marry two sisters. We saw the domestic misery this exact situation caused in the life of Jacob with Leah and Rachel. God here forbids it because it inevitably creates rivalry and strife within the core of the family. It turns sisters, who should be allies, into rivals. This law protects the integrity of both the marriage bond and the sibling bond.
Conclusion: The Gospel and a Rightly Ordered House
It is easy for modern Christians to read a chapter like this and dismiss it as either obvious or irrelevant. But we must not do that. These laws are a profound revelation of God's wisdom and His love for us. He gave these laws because He knows how we are wired. He knows that for human beings to flourish, the family must be protected. When these boundaries are transgressed, the result is not liberation, but devastation, guilt, shame, and confusion.
Our world has rejected every one of these principles. It celebrates sexual confusion as a virtue. The result is a society awash in abuse, heartbreak, and identity crises. The church must be a city on a hill, a place where the beautiful, coherent, and life-giving order of God's design for the family is on full display.
But there is a deeper lesson here. The sin of incest is a picture of a deeper spiritual sin. It is the sin of turning inward, of refusing to go outside the self to find completion. It is a self-referential love that becomes corrupt. This is the essence of all sin. It is a turning in on oneself, a refusal to love God and neighbor. It is trying to find life within the confines of our own flesh.
The ultimate answer to this inward-curving corruption is the gospel. The gospel is the story of how God the Son went outside of the Godhead, outside of heaven itself, to take a bride for Himself, the Church. He did not turn inward; He moved outward in love. He came to cleanse His bride from all her wicked and confused loves, from all her spiritual incest and adultery. He came to wash her and make her holy, so that He might present her to Himself in splendor (Ephesians 5:25-27).
Through faith in Christ, we are forgiven for our participation in this world's sexual chaos, whether in thought, word, or deed. And more than that, we are brought into a new family, the household of God. In this family, the lines are clear. God is our Father, Christ is our elder brother, and we are brothers and sisters together. And in this family, we learn how to love one another with "all purity" (1 Timothy 5:2). We learn the grammar of kinship. We learn to build the walls and fences of holiness, not as a prison, but as the beautiful and protective architecture of a life that is truly free.