Catechism of the Catholic Church

Incest designates intimate relations between relatives or in-laws within a degree that prohibits marriage between them. St. Paul stigmatizes this especially grave offense: "It is actually reported that there is immorality among you... for a man is living with his father's wife.... In the name of the Lord Jesus... you are to deliver this man to Satan for the destruction of the flesh...." Incest corrupts family relationships and marks a regression toward animality. (2356, 2207)