Genesis 19:37

It came to pass on the next day, that the firstborn said to the younger, “Behold, I lay last night with my father. Let’s make him drink wine again tonight. You go in, and lie with him, that we may preserve our father’s family line.”

They made their father drink wine that night also. The younger went and lay with him. He didn’t know when she lay down, nor when she got up.

Thus both of Lot’s daughters were with child by their father.

The younger also bore a son, and called his name Ben Ammi. He is the father of the children of Ammon to this day.