Leviticus 21:3

“And for his sister a virgin, that is nigh unto him, etc.] That is, his sister by both father’s and mother’s side, as Aben Ezra; though, according to Gersom, his sister by his father’s side, and not by his mother’s side, is meant; but, according to Alphes, by his mother’s side: perhaps this may signify not nearness of kin, which is expressed by being his sister, but nearness of place, for, being unmarried, she remained unto her death in her father’s house: which hath had no husband; neither betrothed to one, for then she would have been nigh to her husband, and not her brother, and therefore he might not pollute himself for her, as Gersom observes; nor married to him, for such an one he might…”

“That is nigh him - That is, by nearness not of relation, (for that might seem a needless addition) but of habitation, one not yet cut off from the family. For if she was married, she was now of another family, and under her husband's care in those matters. Leviticus 21:4”