Sforno on Deuteronomy 21:13/ספורנו על דברים כ״א:י״ג

1ובכתה את אביה ואת אמה, to get her father and mother out of her system and reconciling herself to the fact that henceforth she would live in the Land of Israel, have no more contact with her parents. (whether they are dead or alive) Similar wording occurs in Psalms 45,11 שכחי עמך, the psalmist urging the Jewish people, compared to a princess, to forget their former environment and associates as she had been promoted to a loftier position. The prisoner of war described here as embracing Judaism has similarly been promoted to a far superior environment, severing her previous relationships completely. The weeping mentioned in our verse has nothing to do with the death of her parents for we do not kill mothers altogether. She weeps for having no longer a father and mother in the legal sense as these did not convert. Any other convert, not a prisoner of war, would similarly weep for this separation having occurred in her life as a result of her new status.