Rashi on Genesis 34:31/רש"י על בראשית ל״ד:ל״א

1הכזונה implies as one unprotected.

2אחותנו The Targum renders it as an accusative — ית אחתנא “our sister”,) so that the meaning is: Should one (be permitted to) make our sister become as a woman who has no-one to protect her (Genesis Rabbah 80:12).