ויקרא, פרק י״ח, פסוק ט״ו

The prohibition against intimate relations with a daughter-in-law is rooted in the deep bond formed through marriage. When a son marries, his wife becomes an inseparable part of the family fabric, to the point where she is considered the father's own flesh. Harming her is viewed as a direct offense against the son himself. Although she is not a biological relative, violating her intimate boundaries is treated as equivalent to violating the son's own [רלב״ג, ביאור יש״ר].

The Hebrew concept of a daughter-in-law reflects ideas of completeness and inclusion. A son builds his father's house and ensures the continuity of the family through his wife. She is given this specific title because she perfects and completes the home, a role defined precisely through her relationship with her parents-in-law [ביאור יש״ר]. This status applies to a woman whether she is betrothed or fully married [פירושי רד״צ הופמן]. However, it is strictly limited to a direct daughter-in-law, not the wife of a grandson, whose restriction stems only from later rabbinic rulings [תורה תמימה].

The legal definition of this relationship relies on two distinct descriptions, identifying the woman as both a daughter-in-law and a son's wife to carefully set the boundaries of the restriction. The primary approach among commentators is that identifying her as the son's wife ensures the prohibition only applies when a valid, legal marriage exists. This condition excludes a woman the son may have assaulted or seduced outside of wedlock, as well as a maidservant or a foreign woman with whom a legal marriage cannot be contracted [רש״י, בכור שור, ברכת אשר, פירושי רד״צ הופמן].

While other forbidden relationships might include women taken by force, placing the broader concept of a daughter-in-law alongside the restrictive condition of being a son's wife shows that the two ideas explain each other. A forbidden daughter-in-law is strictly defined as one who is legally married to the son [מזרחי, שפתי חכמים, גור אריה]. By pairing these concepts, it becomes clear that both descriptions represent the exact same legal standing [מזרחי, הכתב והקבלה].

This dual phrasing also clarifies the status of the relationship if the son passes away. Commentators agree that the prohibition is permanent and does not expire upon the son's death [הכתב והקבלה, פירושי רד״צ הופמן]. Yet, there is an interesting discussion regarding how this permanent status is derived. Some suggest that she is a daughter-in-law while the son is alive, and the added description of being the son's wife extends the restriction after his passing. Others argue the exact opposite, noting that being a son's wife implies an active marriage during his lifetime, while the title of daughter-in-law remains with a woman forever. This enduring title is evident in historical accounts of women like Tamar and Ruth, who were still referred to as daughters-in-law long after they were widowed [פרדס יוסף].