Комментарий к Берешит 19:38

והצעירה...בן עמי, meaning “the father of this child was a member of my family.” This name was somewhat more chaste than the name given by her older sister, as she did not spell out that Lot was the father of her child.

והצעירה גם היא, “and the younger sister did also give birth to a son;” the reason this is reported in a separate verse, is to signal that her son too would be originator of a family destined to produce kings. A distant descendant of her was Naamah who became the mother of Rechavam, son of Solomon (Talmud Baba Kamma, 38).