Sforno on Genesis 19:37/ספורנו על בראשית י״ט:ל״ז

1ותקרא שמו מואב...ותקרא שמו בן עמי. They called the names of their children thus to show the world that the father was not someone who was an unworthy, sinful person. [from the point of view of Jewish law as it pertains to gentiles, a father may sleep with his daughter. Ed.]

2הוא אבי מואב...הוא אבי בני עמון, the ones who would in a short while inherit large parts of what would eventually be part of the land of Israel on the east bank of the Jordan river. Seeing that the intentions of both daughters in sleeping with their father had been honourable, they were rewarded with founding two nations, both from the same father. (Nazir 23) In a certain sense they had become heirs of Avraham to whom all the Canaanite lands had been promised. This reflects the statement in Proverbs 3,6 בכל דרכיך דעהו, “know (try to emulate) G’d in all your undertakings;” (even when involving an apparent sin. [based on Berachot 63. Ed.]