וְלַאֲחֹת֤וֹ הַבְּתוּלָה֙ הַקְּרוֹבָ֣ה אֵלָ֔יו אֲשֶׁ֥ר לֹֽא־הָיְתָ֖ה לְאִ֑ישׁ לָ֖הּ יִטַּמָּֽא׃
dan untuk saudara perempuannya yang masih perawan, yang dekat dengannya, yang belum pernah menjadi milik seorang laki-laki — untuk dialah ia harus menajiskan dirinya.
Siftei Chakhamim
But perhaps to a “nesuah.” Meaning that although the plain understanding of הויה [lit. "being" having the connotation of marriage] refers to kiddushin [betrothal], nonetheless, we also find הויה referring to relations as in “his virgin [sister] who is close to him, who was not היתה ["married"] to a man” (Vayikra 21:3) where [היתה] refers to relations, meaning she was a nesuah. Thus, I would have said that הויה here also refers to relations and a she was a nesuah, such that both the father and the husband would annul her vow. However “when it says…”