seme is so good looking but he's a product of generations of incest ???

Yeah apparently so,and I'm just now finding out after rereading it for the 100th time

I mean it is already suggested since he is a king, the story is surprising historically accurate compared to a lot of historical stories here. Historically speaking, a lot of rulers were inbred as they believed royalty they should keep their "amazing" genetic or blood or bloodline to themselves, hence why they kept marrying cousins to cousins or sometimes even uncles to nieces.