Affinity is kinship by marriage, not by shared blood
In law and anthropology, affinity links each spouse to the other’s relatives through marriage. Unlike consanguinity, it is a social and moral construct—sometimes with hard legal teeth. Rules vary: some affinities end at death or divorce; incest and marriage bans may still treat affines as off-limits.
Affinity covers each spouse's relationship to the other's family but not the marriage bond itself. Depending on the tradition it may end when the linking spouse dies or when the couple divorces, and the term sometimes stretches to step relationships and adoption, though law usually handles those as if they were blood ties. Blood kinship can carry genetic consequences; affinity is a social and moral arrangement that the law sometimes enforces.
Its legal bite shows up in incest rules and marriage bans, which differ between jurisdictions, have shifted over time and in many places once rested on religious law. Degrees of affinity copy the blood degree inside the marriage: a husband stands in the second degree to his sister-in-law, as his wife does to her sister, the first to her parent or child, the third to her aunt or niece and the fourth to her first cousin.
Everyday English marks these relatives with in-law for close links such as parent-in-law or sibling-in-law, but usually drops it further out. Because uncle and aunt are also loosely used for family friends, they often go unmarked, and a cousin's spouse may be called a cousin by marriage or not counted at all; Princess Léa of Belgium, for instance, is an aunt by marriage of King Philippe.
South Africa forbids sexual relations in the first degree of affinity, between someone and a direct ancestor or descendant of their spouse. Brazil's Civil Code, Article 1521, bars marriage between parents- and children-in-law even after divorce, and between stepparents and stepchildren. Hawaii punishes marriage or sex within close affinity with up to five years in prison, and in Michigan sexual contact with blood or affinity relatives to the third degree can bring two years or a $500 fine. New Jersey applies a similar third-degree rule when the victim is 16 or 17.
Source: Affinity (law)
