The line of Targaryens ruling in King's Landing started from Aegon the Conqueror fucking his two sisters. Incest is a core family tradition, even long before the events of that show.

Yeah, surprisingly Martin understood (at the time) that races hail from different lands and don't just exist all willy nilly across the world, especially in a time period where mass transit is impossible for normies.

The niggles should have never been in the show, and I'm glad they're getting written off.