ויקרא, פרק י״ח, פסוק ח׳

Family boundaries form the foundation of a moral society, particularly regarding the sacred relationships within a household. A strict boundary is drawn around a father's wife, ensuring respect and purity within the family structure. The primary approach among commentators is that this specific prohibition refers to a stepmother, meaning a wife of the father who is not the biological mother, as the biological mother was already addressed earlier.

The commentators expand the boundaries of this prohibition to include a stepmother who is only engaged to the father. In Jewish law, an engaged woman is treated as a full wife from the moment of betrothal [תורה תמימה, רלב״ג, פרדס יוסף]. However, a woman whom the father merely assaulted or seduced is not included, as she is not considered his legal wife [רלב״ג]. Even so, there is a viewpoint noting a dispute over whether a strict biblical prohibition still applies in these non-legal cases [בכור שור].

The laws surrounding a stepmother carry intertwined legal, conceptual, and moral meanings. From a legal standpoint, since the prohibition of relations with a stepmother during the father's lifetime is already established, the additional emphasis teaches that this ban remains entirely in force even after the father passes away [רש״י, תורה תמימה, בכור שור, ברכת אשר].

Conceptually, violating this boundary is an act of deep disgrace. Engaging in relations with a father's wife directly damages the father's honor and causes him profound shame [ביאור שטיינזלץ]. The woman is forbidden to the son precisely because of her intimate bond with his father [רש ר הירש]. Expanding on this moral connection, some explain that a person's second marriage is determined by his deeds and character. Therefore, harming the father's second wife is an attack on the very essence of the father. Furthermore, a son's severe moral failure with his stepmother could bring slander upon the father himself, leading people to falsely assume that the father must also engage in immoral behavior [פרדס יוסף].

Regarding the broader context of these laws, there is a perspective that the rule about a father's wife serves to clarify a vague reference to the father made earlier. In contrast, another approach argues that the earlier reference specifically forbids relations with the father himself, making the prohibition regarding the father's wife a completely separate and distinct law [בכור שור].