How to make characters marry inside only their culture group?

[Rognvaldr19]

Getting fed up of characters (especially kings and emperors) swiping left on good candidates for marriage in their own lands but thinking candidates with even less standing or inheritance hundreds of miles away from them is a great idea. I recently just had a game where Charlemagne decided to marry an Irish count's daughter for example and then his son married a bedouin catholic, despite there being dozens of young unmarried Nibelunging women.

I want to stop that happening, so is there a way to mod the game so they will at least marry within there culture group?