SPOILER ALERT: I'm divulging plot points in Wolf Hall by Hilary Mantel. Beware.
Incest was a real concern in the medieval and early modern periods, and it was much more stringently defined than it is in modern Western countries. In the US, it is sometimes illegal for first cousins to marry, but most of the taboo centers on sibling, parental, or grandparental incest (also aunts and uncles, though I'm not sure what the term is for that). While it's currently illegal in many states for relatives in the first degree to marry (first cousins, or an aunt and a nephew), medieval and early modern Church law was far stricter, often prohibiting marriage between first, second, and third cousins, as well as relatives by marriage. For most people, this wasn't an issue; the vast majority of people got married and had kids, sometimes doing the latter before the former, and sometimes to people within their extended kin group. But for the well-born and the elite, papal dispensations for consanguinity were often required and sought before marriage took place.
This explains why incest is such a common theme in medieval and early modern literature and culture. King Arthur has an incestuous relationship with his half-sister, Morgaine. One medieval biography of Charlemagne states that he loved his daughters so strongly that he couldn't bear to see them married (coy, but effective). When the family group extends far beyond the nuclear family, encompassing foster- and step-relations, half-siblings and distant cousins, as it did in the medieval period, the definition of incest--sex within one's kin group--expands, as well.
Incest is also common in historical fiction set in the medieval and early modern period, like Wolf Hall. The story is familiar and often-told: Henry VIII's divorce from his first wife, Katherine of Aragon, and his subsequent marriage to Anne Boleyn. But the protagonist is the modern, self-made man Thomas Cromwell, Henry's advisor and chancellor, instead of the humanist scholar and man for all seasons, Thomas More.
Yet what struck me the most was that the book is all about incest. Incest, incest, incest. Thomas Cromwell has an affair with his dead wife's (married) sister. Thomas More is accused (by his wife, Alice) of schtupping his beloved daughter, the intelligent, educated, principled Margaret (legend has it that after More was executed as a traitor and his head stuck on a pike outside the Tower, she shimmied up the pole under the cover of darkness and retrieved his head so that it could be buried with the rest of his body). Jane Seymour's father scandalizes the court by having an affair with his daughter-in-law. Mary Boleyn, Anne's sister, is Henry's mistress before Anne.
All of this, of course, echoes the main incest plot that drives the entire story: Henry's marriage to his brother's widow, Katherine of Aragon. Despite Katherine's insistence that her first marriage to Arthur was unconsummated (rendering that marriage invalid) and the papal dispensation after Arthur's death that allowed Henry to marry his brother's widow, Henry is convinced that he has violated God's law with his incestuous union, taking as proof of divine displeasure his lack of a male heir. He pursues a divorce after almost two decades of marriage to his queen.
The incest motif continues beyond the time frame of Wolf Hall. One of the charges brought against Anne Boleyn was that her brother, George, was one of her many lovers. Mantel hints at this accusation in Wolf Hall; perhaps she'll take it on in her follow-up novel.