ויקרא, פרק כ״א, פסוק ג׳

The Torah establishes strict boundaries to preserve a priest's holiness by restricting his exposure to ritual impurity from a corpse. However, it simultaneously honors deep family bonds, requiring him to become impure for his immediate relatives. In the case of his sister, whether paternal or maternal [תורה תמימה, חזקוני, אבן עזרא], this permission and obligation depend entirely on her personal and family status. She must be a virgin, a requirement that deliberately excludes a sister who was assaulted or seduced; in such tragic cases, the priest may not become impure for her [תורה תמימה, רד צ הופמן, אדרת אליהו, ביאור יש״ר]. While commentators debate the exact categorization of a mature unmarried woman or a sister who lost her virginity accidentally, they agree that the priest is ultimately permitted to become impure for them [מלבי ם בשם ר' מאיר ור' יהודה, מלבי ם בשם ר' יוסי ור' שמעון, ברטנורא, אדרת אליהו].

The requirement that the sister be closely dependent on him reflects a historical social reality in which an unmarried or orphaned sister relied upon her brother's household for support until her marriage [העמק דבר, חזקוני]. The primary approach among commentators is that this proximity includes a betrothed sister. Because she has accepted betrothal but has not yet relocated to her husband's home, her marriage is incomplete, and she remains intimately tied to her brother's family [רש״י, מזרחי, גור אריה, משכיל לדוד, ברכת אשר]. This enduring closeness also applies to a woman whose betrothal ended in divorce before full marriage, as well as a mature sister who continues to live in her father's house [תורה תמימה, מלבי״ם, אדרת אליהו].

The stipulation that the sister never had a husband is interpreted by most commentators as referring to the absence of physical consummation, rather than merely the absence of betrothal [רש״י, מזרחי, ריב״א, שפתי חכמים, ביאור יש״ר]. This explicitly excludes a fully married sister. Once she has been married, her brother may no longer become impure for her, even if she subsequently divorces [רלב״ג, הכתב והקבלה]. The underlying logic is that a married woman has established her own family unit; she presumably has a husband or children who will take responsibility for her burial, thereby relieving her brother of the obligation [שד״ל].

Becoming impure for a sister is not merely an optional concession; it is an absolute Commandment. If a priest refuses to defile himself for his immediate relatives, he is actively compelled to do so. This echoes a historical incident where the sages physically forced a priest named Yosef to become impure for his deceased wife after he refused [רש״י, מזרחי, תורה תמימה, רש ר הירש, חזקוני, ברטנורא]. Nevertheless, this stringent duty is bound by precise limitations. First, the priest may only become impure when the family relationship and the state of impurity are absolutely certain, entirely excluding cases of doubt [תורה תמימה, צפנת פענח, אדרת אליהו]. Second, his involvement is restricted solely to his sister. He is forbidden from using this permitted exposure as an opportunity to bury other dead or gather unrelated remains [תורה תמימה, חזקוני, ברטנורא]. Finally, he may only become impure for her actual remains after her death, but he is strictly prohibited from becoming impure for a limb amputated while she was still alive [בכור שור, חזקוני, אדרת אליהו].