The Roman Catholic Church's definition of incest is one of the most intriguing features of medieval marriage. Neither the Old nor New Testament provided any basis for it. But in the mid-sixth century, church synods began to denounce as incestuous the Old Testament practice of marrying a brother's widow. Also, during the sixth and seventh centuries bishops began condemning marriage to first and second cousins, stepmothers or stepdaughters, and the widows of uncles. [...]
A few decades later marriage was forbidden up to the seventh degree of separation [...]. This made it illegal to marry a descendant of one's great-great-great-great-grandfather! By the end of the eighth century it was incestuous to marry in-laws, the kin of godparents or godchildren, or a relative of someone you had once had sexual intercourse with. It was also forbidden to marry a relative of someone you had previously promised but failed to marry. These prohibitions were so broad that almost any match could be ruled invalid. One historian notes that, at least in theory, the incest rules prohibited young village men from wedding "all the marriageable girls they could possibly know and a great many more besides."
Whatever the reasons for their breadth, these incest prohibitions became very useful weapons in the power struggles of the age. [...][I]n the medieval period the Church still enforced most of its principles on marriage erratically and arbitrarily. It spent little time investigating the marriages of common folk, although it readily sold dispensations when a conscientious commoner asked for a formal exemption from the rules that most people simply ignored. Even in a royal or aristocratic marriage, the Church seldom inquired into the degree of familial connection unless it was engaged in a power struggle with one of the families involved or was asked to intervene.
[...] In 1215 the Fourth Lateran Council narrowed the definition of incest to four degrees of separation. The council's stated aim was to enforce the modified ban more stringently. But popes continued to grant dispensations for political or financial gain. Under the papacy of Boniface IV (1389-1404), marital dispensations were openly available for sale, with a sliding fee scale based on the value of the concession being sought.- Stephanie Coontz, Marriage, a History