Rashi on Leviticus 21:3/רש"י על ויקרא כ״א:ג׳

1הקרובה [AND FOR HIS SISTER A VIRGIN] THAT IS NIGH UNTO HIM — This is intended to include the sister who was only betrothed and has therefore not yet left her father's house and who is consequently still near to him (Sifra, Emor, Section 1 1; Yevamot 60a), for it continues

2אשר לא היתה לאיש, WHO HATH HAD NO HUSBAND — i. e. who has not yet entered into marital relations with him:

3לה יטמא — This is a command, [and the translation is: FOR HER HE SHALL DEFILE HIMSELF, not, “he may defile himself”] (Sifra, Emor, Section 1 12; Zevachim 100a).