Catechism of the Catholic Church §2388 — excerpt
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 .
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.