This pattern is well known historically.   
      
   As cultures become more complex and stratified; related people tend to   
   marry relatives to maintain social/political/economic power within kinship   
   groups. .   
      
   Europe has had many first cousins marry to cement ties between royal   
   families..  Queen victoria and her husband are an example.   
      
   Ancient egypt had brother/sister first degree marrage.   
      
   Aztecs in mexico did likewise.   
      
   Here is an example known by looking at genetics.   
      
   First-degree incest: ancient genomes uncover Irish passage tomb dynastic elite   
      
   https://www.eurekalert.org/pub_releases/2020-06/tcd-fia061520.php   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)