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 You shall not uncover the nakedness of your sister, your father’s daughter or your mother’s daughter, whether brought up in the family or in another home.