@PotomacBobThe UK incest statute
Incest by a man
(1)It is an offence for a man to have sexual intercourse with a woman whom he knows to be his grand-daughter, daughter, sister or mother.
(2)In the foregoing subsection " sister " includes half-sister, and for the purposes of that subsection any expression importing a relationship between two people shall be taken to apply notwithstanding that the relationship is not traced through lawful wedlock.
11Incest by a woman
(1)It is an offence for a woman of the age of sixteen or over to permit a man whom she knows to be her grandfather, father, brother or son to have sexual intercourse with her by her consent.
(2)In the foregoing subsection " brother" includes half-brother, and for the purposes of that subsection any expression importing a relationship between two people shall be taken to apply notwithstanding that the relationship is not traced through lawful wedlock.
In a number of countries in Europe (Spain and Portugal for example) incest is not illegal for persons above the age of consent. In France, it may or may not be illegal - it wasn't, but there was a movement to change the law.
In none of these countries is marriage with close relatives permitted.
Generally in the US, incest is illegal with ancestors, deescents and siblings. Most places aunts and uncles are included.
In NJ adult incest is legal, in RI incest above the age of consent is allowed. In Ohio, the ban on incest only applies to parents/children or people in loco parentis. Marriage to parents and siblings is illegal everywhere.