-- 1 Thes 4:1-8
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.
-- CCC 2388