That's good, its effect should be increased.There is already code that checks the distance between clans as a part of their suitability.
Sure, nobody wants to break the game, but it can be better than it is now without posing too much risk, and if it causes problems adjustments can be made as needed (eg: reduce the amount which clans lose relation, or decrease the importance of relation in marriage matching slightly).I'm not sure how it handles fiefless clans but there aren't many of those floating around. It is setup to favor marriages happening over not happening though, presumably for world state reasons; players would ***** mightily if clans started going extinct because they were hated by every other clan in their faction (voting against them consistently) and all their immediate neighbors (raids and conquest) so there were no suitable matches for their eligible members.
Battanians and Khuzaits are heavily based on cultures who were on opposite sides of Europe, not the distance between Turkey and Armenia.Anyway, Calradia's faction have full knowledge of the others and plenty cross-cultural contact so it isn't an equivalent to your example. Instead it is more like someone from Thrace marrying someone from the Black Sea steppes, which definitely did happen. I can't speak to other people's immersion but cross-cultual marriage among nobility wasn't uncommon historically.
From an in-game perspective Battanians know of Khuzaits and vice versa (Khuzaits being brought along as mercenaries at Pendraic), but "full knowledge" is questionable. Either way the in-game lore matters less than the real-life inspiration since most players tend to heavily identify factions with their real-life counterparts.