וְלַאֲחֹת֤וֹ הַבְּתוּלָה֙ הַקְּרוֹבָ֣ה אֵלָ֔יו אֲשֶׁ֥ר לֹֽא־הָיְתָ֖ה לְאִ֑ישׁ לָ֖הּ יִטַּמָּֽא׃
and for his sister a virgin, that is near unto him, that hath had no husband, for her may he defile himself.
Read kabbalistic (mystical) interpretation of Leviticus 21:3: and classical mystical sources.