One day the firstborn told the younger one, "Our father is old, and there's no man in the land to have sex with us, as everybody else throughout all the earth does.
So they had their father drink wine that night, and the older one had sexual relations with her father, but he was not aware when she lay down or when she got up.
The next day the firstborn told the younger one, "Look! I had sex with my father last night. Let's make him drink wine tonight again as well. Then you have sex with him, too. That way we'll preserve our father's lineage."
The firstborn gave birth to a son and named him Moab, and he is the ancestor of the Moabites to this day.
1 Samuel 14:49
Saul's sons included Jonathan, Ishvi, and Malchi-shua. Of his two daughters, the firstborn was named Merab, and the younger one was named Michal.