1) *Canon law was strict in forbidding marriage between couples too closely related by marriage or by being a godparent. Such marriages could be rendered null by the Church. Thus, though a later instance, Henry VIII's marriage to Catherine of Aragon was deemed null because she had allegedly been married to Henry's brother, Arthur. When, in the 11c and 12c, the Latin affinitas was used it was this sense that was intended, not the second political sense found in the following entry. [< Lat. affinitas = relationship] -
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2) Affinity is used today to describe the network of relations within a magnate's own country. This comprised knights and esquires who supported him in local affairs. During Edward III's reign, and the emergence of *bastard feudalism, magnates began to take on permanent retainers; they were also extending their power and were able to influence local courts through which they intimidated those they wished to constrain. While the *justice-in-eyre system might bring central authority to justice, local politics remained comfortably within the magnate's hands.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.