Short answers, pulled from the story.
Affinity is the kinship relationship created or that exists between two people as a result of someone's marriage. Unlike blood relationships known as consanguinity which may have genetic consequences, affinity is essentially a social or moral construct.
Sometimes these relationships cease with the death of one of the marriage partners through whom affinity is traced. In other cases they end with the divorce of the marriage partners.
The degree of affinity is considered the same as the consanguineal level of the couple that was joined. For example the degree of affinity of a husband to his sister-in-law is two while the degree to the wife's parent or child is one.
In South Africa sexual relations are prohibited within the first degree of affinity under the Criminal Law (Sexual Offences and Related Matters) Amendment Act 2007 section 12. Brazilian law extends invalidity of marriage between parents and children to grandparents and grandchildren via Article 1521 of the Civil Code.
In Hawaii sexual penetration and marriage is prohibited within close degrees of affinity and this offense is punishable by up to 5 years in prison. In Michigan sexual contact between persons related by blood or affinity to the third degree is chargeable as criminal sexual conduct in the 4th degree involving a 2-year sentence or a fine of up to $500.