Sexual intercourse between persons too closely related for normal marriage. The twofold theological rationale for the prohibition of incestuous unions is the divine claim “I am the LORD your God” (Leviticus 18:2,Leviticus 18:4,Leviticus 18:6) and the note that such behavior characterized the Egyptians and Canaanites whom God judged (Leviticus 18:3,Leviticus 18:24-25).
Leviticus 18:6-16 prohibited unions between a man and his mother, stepmother, sister, half-sister, daughter-in-law, granddaughter, aunt (by blood or marriage), or sister-in-law.
Leviticus 18:19 prohibited a man's involvement with a woman and her daughter or granddaughter.
Leviticus 18:18 prohibited taking sisters as rival wives. Penalties for various forms of incest included childlessness (Leviticus 20:20-21), exclusion from the covenant people (Leviticus 18:29;
Leviticus 20:17-18; compare
1 Corinthians 5:2,1 Corinthians 5:5), and death (Leviticus 20:11-12,Leviticus 20:14). In patriarchal times marriage to a half sister (Genesis 20:12) and marriage to rival sisters (Genesis 29:21-30) were permissible, though such marriages proved troublesome to both Abraham and Jacob. Scriptural accounts of incest include
Genesis 35:22; and
2 Samuel 13:1.