"She really is my sister, the daughter of my father though not of my mother; and she became my wife." (Gen 20:12)
"...so both of Lot's daughters became pregnant by their father." (Gen 19:36)
"Some time later Abraham was told, 'Milcah is also a mother; she has borne sons to your brother Nahor: Uz the firstborn, Buz his brother, Kemuel (the father of Aram), Kesed, Hazo, Pildash, Jidlaph and Bethuel.' Bethuel became the father of Rebekah." (Gen 22:20-23)
It was honestly very common practice then and also in recent history. Things just change with the times and I was describing my annoyance when people use scriptures and say "YOU MUST OBEY" but only obey some scriptures while ignoring others.