ויקרא, פרק י״ח, פסוק ט׳

The prohibition against incest with a sister establishes an absolute and sweeping boundary, leaving no room for family or social exceptions. The blood tie between siblings creates a severe and permanent restriction, entirely independent of the parents' marital status or the circumstances surrounding the sister's birth. This applies equally whether she is a full sister or a half-sister sharing only a father or only a mother [ביאור יש״ר, רד״צ הופמן, שטיינזלץ]. Furthermore, a paternal sister includes a daughter born from rape or seduction, not merely those born within the confines of marriage [רש״י, מזרחי, גור אריה].

This sweeping scope is further emphasized by addressing sisters born both within the household and outside of it, a distinction that commentators explore through several lenses [רש״ר הירש, ביאור יש״ר]. The primary approach focuses on the marital framework. A sister born at home refers to one born from a legitimate marriage, where the father lawfully brought the mother into his household. Conversely, a sister born outside refers to one born out of wedlock, whether through prostitution, rape, or seduction [רמב״ן, רלב״ג, בכור שור, שטיינזלץ]. Both scenarios are explicitly detailed to dispel the misconception that children born outside of marriage lack familial status. It underscores that the bonds of kinship and the ensuing prohibitions apply equally, regardless of illegitimacy [רמב״ן, ביאור יש״ר].

Another perspective examines the legal validity of the parents' relationship. A sister born at home is the product of a permitted union. In contrast, a sister born outside is the result of a forbidden relationship that requires the parents to separate. Even if the parents' union violates severe prohibitions or lacks legal validity, the resulting daughter remains a sister and is strictly forbidden [רש״י, מזרחי, גור אריה, חומש קה״ת].

A third viewpoint distinguishes between paternal and maternal ties. Because the father is considered the foundation of the household, a sister born at home is a paternal sister, even if born to another woman or from rape. A sister born outside is a maternal sister fathered by another man, reflecting the reality that a woman often leaves her first husband's home after his death to remarry elsewhere [נתינה לגר, חזקוני, שד״ל]. Finally, a physical distinction can be made regarding their upbringing: one sister may have been raised alongside her brother in the same house, while another was raised elsewhere [העמק דבר].

Despite the vast expansion of this prohibition, traditional oral law dictates specific exceptions. A sister born to a maidservant or a non-Jewish woman is excluded from this restriction, as lineage follows the mother in these cases and a formal marriage cannot take effect [רמב״ן, מזרחי, שפתי חכמים]. Similarly, a stepsister—the daughter of a father's wife from a previous marriage who shares no blood relation—is entirely permitted. This remains true even if she was raised in the same home and is colloquially referred to as a sister [העמק דבר]. Ultimately, the law concludes with a sweeping declaration, emphasizing that the prohibition applies to each and every type of sister mentioned, without exception [ביאור יש״ר].