Unmentionable

Another theme of Wolf Hall is incest, especially as defined by canonical law. Henry VIII became convinced that his lack of a son with Katherine of Aragon was a sign of divine displeasure at his having married his brother's widow. This was a forbidden marriage by Roman Catholic canonical law at that time (I don't know about today), and therefore a pope had to grant a dispensation for it. When a later pope would not annul the marriage, Henry VIII took the stand that the pope had had no right to overrule canonical law in the first place, and furthermore that the pope had no authority in England, being merely the "Bishop of Rome," while (Henry and his legislators alleged) the papacy was a later corruption of an earlier tradition in which the sovereign also was the religious leader of his people.

Anyway, Henry thought marrying his brother's widow was incest. Later, one more of many reasons he found to discard Anne Boleyn was that their marriage had also been incestuous because Henry had previously had an affair with her sister, Mary Boleyn (their affair is a known historical fact; there were also rumors that, earlier yet, Henry had had an affair with the Boleyn sisters' mother, but Henry himself denied that).

Then, Anne was accused of incest with her brother when the king wanted reasons to execute and defame her. Also the novel's plot gives credence to a rumor that has survived (the truth of which is unknown) that Jane Seymour's father had an affair with the wife of his son, Edward Seymour. Finally, in the fictional world of the novel, Thomas Cromwell has an affair with his deceased wife's sister, which I don't think has ever been speculated by historians to have occurred.