Do Not Have Relations with One’s Sister

Full Sister and Half-Sister — The Breadth of the Prohibition

Leviticus 18:9: “Do not have sexual relations with your sister, daughter of your father or your mother, whether born at home or born outside; do not have relations with them.” The verse deliberately covers every permutation: paternal half-sister (same father, different mother), maternal half-sister (same mother, different father), and both household-born and born outside — a phrase the rabbis interpreted to include children born out of wedlock, illegitimate children who share a bloodline.

The prohibition derives from the principle that blood creates erva, regardless of legal marital status. A half-sister born of the same father but a different mother is equally prohibited as a full sister. The Torah’s word “achotecha” (your sister) is defined by the verse itself: any woman who shares either parent. The Talmud (Yevamot 97b) extends this analysis to discuss cases of uncertain paternity.

Amnon and Tamar — The Paradigm Violation

2 Samuel 13:1–14: Amnon, David’s firstborn, became obsessed with Tamar, his half-sister (both David’s children, different mothers). His friend Jonadab devised a scheme; Amnon feigned illness and requested that Tamar bring him food. Alone with him, he assaulted her. The text records Tamar’s words: “No, my brother, do not violate me, for such a thing is not done in Israel—do not do this disgraceful thing” (2 Sam 13:12).

Tamar says something remarkable: “Please speak to the king, for he will not withhold me from you” (2 Sam 13:13). She appears to believe David might permit such a marriage. The rabbis grappled with this: Sanhedrin 21a suggests Tamar was the daughter of a Canaanite captive woman whom David had married, making her technically Amnon’s half-sister by maternal line but not yet a full Israelite — a legal edge case that was later closed. Whatever the legal ambiguity, the narrative frames it unambiguously as a violation and a disgrace.

Absalom, Tamar’s full brother, waited two years and then killed Amnon (2 Sam 13:28–29). The narrative consequence is devastating: the rape of Tamar sets off the chain of events that would divide the Davidic house, produce Absalom’s rebellion, and end in multiple deaths. The Torah prohibition against sister-relations is not merely ceremonial — its violation set Israel’s royal line on a path of destruction.

Before the Law — Abraham and Sarah

Genesis 20:12: Abraham says to Abimelech: “Moreover, she is indeed my sister, the daughter of my father though not the daughter of my mother; and she became my wife.” Abraham married a woman who was his half-sister by the paternal line — exactly what Leviticus 18:9 prohibits. The Torah records this without censure because, as with Amram and Jochebed (see commandment #521), the Sinaitic revelation replaced a less restrictive pre-Sinai framework.

The rabbis (Sanhedrin 58b) note that the Torah’s prohibitions were given at Sinai and are not retroactively applied to the patriarchs. Abraham’s marriage to Sarah represents a pre-Sinaitic relational structure that Sinai prohibited going forward. The same logic applies to Amnon’s case, except Amnon lived after Sinai — which is precisely why it is called a disgrace.