Consanguinity.
the fact of being descended from the same ancestor.
"the marriage was annulled on grounds of consanguinity"
This word comes up frequently in my studies of English/French history of the 11th century. It was difficult for royalty to get a divorce, but in the case of the young Eleanor of Aquitaine, she was able to get her first marriage to Louis 7 King of France, annulled on the grounds of consanguinity within the fourth degree .. Their daughters were declared legitimate, custody was awarded to Louis, and Eleanor's lands were restored to her and she became free to marry the 10-years-her-junior-super-hot Henry Plantagenet soon to be Henry II King of England and France. And the rest,as they say..is history...
.(Lion in Winter with Katherine Hepburn and Peter O' Toole was based on their rather tempestuous marriage and ensuing family dramarama..)
think Henry and Eleanor as Arthur and Guenevere in the musical Camelot.