In law and in cultural anthropology, affinity is the kinship relationship created or that exists between two people as a result of someone's marriage.
This is standard for the closest degrees of kinship, such as parent-in-law, child-in-law, sibling-in-law, etc., but is frequently omitted in the case of more extended relations.
As uncle and aunt are frequently used to refer indifferently to unrelated friends of the family, the terms may be used without specifying whether the person is a cognate or affine.
[2] Brazilian law, by the Article 1521 of the Civil Code, also extends the invalidity of marriage between parents and children to grandparents and grandchildren or any other sort of ascendant-descendant relationship (both consanguineous and adoptive), parents-in-law and children-in-law even after the divorce of the earlier couple, as well as to stepparents and stepchildren, and former spouses to an adoptive parent who did this unilaterally (regarded as an equivalent, in families formed by adoption, to stepparents and stepchildren); and extends the invalidity of marriage between siblings to biological cousin-siblings.
[3][4] In Hawaii, sexual penetration and marriage is prohibited within close degrees of affinity and is punishable by up to 5 years.