In BBC History, Richard III Special Edition, Professor Hicks returns to his theory that Richard III’s marriage to Anne Neville was incestuous because of the prior marriage of his brother, George Clarence, to Isabel Neville.
I have to confess to surprise that a historian of Professor Hicks’ fame and academic stature is still chasing this particular cat down the alley. He must surely be aware from his extensive reading that such marriages were not uncommon in the later middle ages.
For example, Edmund of Langley married Isabel of Castile, despite the undoubted fact that his brother, John of Gaunt, was already married to her sister, Constance of Castile.
In the 1430s, Richard Neville (later to be the ‘Kingmaker’) married Anne Beauchamp. At roughly the same time (possibly on the same day, I don’t remember) his sister Cecily, or Cecille, married Anne’s brother, Henry Beauchamp, Lord Despenser, later Duke of Warwick.
These are two relatively famous examples. There were plenty of similar cases lower down the social scale.
Were Edmund of Langley and Warwick the Kingmaker incestuous and their children illegitimate? Were their parents really so careless when arranging their marriages? I think we should be told.
See also this Marie Barnfield article. Affinity does not beget affinity. QED.