ויקרא, פרק י״ח, פסוק י״א

Biblical laws regarding forbidden relationships often seem to overlap, but close study reveals that these repetitions serve to define exact legal and familial boundaries. A prior command already outlaws relations with a sister, whether from the father's side or the mother's side. The return to this subject refines the limits of the prohibition by distinguishing between types of family relations, legal status, and marriage bonds. Some commentators explain that the earlier prohibition dealt with a sister born outside of a formal marriage, such as from assault or seduction by the father. The current law expands the warning to include a sister born within a legal marriage [רלב״ג, אבן עזרא, חזקוני, רד״צ הופמן]. Another approach, based on Talmudic tradition, suggests the repetition teaches a legal consequence: if a man engages in relations with a sister who is also the daughter of his father's wife, he violates two distinct prohibitions and faces a double penalty [תורה תמימה].

The prohibition specifically targets relationships stemming from a woman who is legally eligible to marry the father. This legal definition excludes a sister born to a maidservant or a foreign woman. Because a valid marriage cannot take effect with them, the child's status follows the mother. Therefore, legally, that child is not considered the father's daughter or the son's sister, and the specific prohibition regarding a sister does not apply [רש״י, ביאור יש״ר, חומש קה״ת, בכור שור, ברכת אשר, רד״צ הופמן].

Conversely, the law explicitly permits a stepsister. If a father marries a woman who already has a daughter from a previous relationship, that daughter is fully permitted to the son. She is not the father's offspring, and the two share no blood tie [תורה תמימה, ביאור יש״ר, ביאור שטיינזלץ, רד״צ הופמן]. In clarifying this, commentators reject the Karaite view, which attempted to forbid a stepsister simply because she is raised and educated in the same household. Such an interpretation contradicts established Jewish law [אבן עזרא, רד״צ הופמן].

An additional emphasis on the sibling relationship might seem unnecessary, but it carries both legal and historical significance. Legally, it establishes the principle that formal warnings and punishments cannot be derived merely through logical deduction. Even if common sense dictates that relations with a full sister are a greater offense than with a half-sister, the Torah must still state the prohibition explicitly [תורה תמימה, חזקוני]. Historically, prior to the giving of the Torah, ancient societies often only forbade relations with a maternal sister, while permitting a paternal one. The law therefore makes a definitive, groundbreaking statement: a sister from the father alone is a sister in every respect, and marriage to her is absolutely forbidden [רד״צ הופמן].