The Lord prohibits sexual relationships between blood-relatives within the immediate family. This includes sexual activity with one’s mother (Leviticus 18:7) or one’s sister (Leviticus 18:9). One’s half-sister is also included in this list (Leviticus 18:11). Wenham calls this, first-degree consanguinity.
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9 Tu ne découvriras point la nudité de ta sœur, fille de ton père ou fille de ta mère, née dans la maison ou née hors de la maison.