
I don't like Incest but I can understand that once it (incest) was allowed in the royal family in many different nations to keep the power stable and bloodline pure (or it was what they thought)
I remember when I watched k-drama Moon Lovers I was shocked seeing princess (forgot her name) to chase after and marry her half brother (the 4th prince) and also another prince (also half brother) to lust after the princess..

Moon Lovers Ending Spoiler:
Hae Soo abandons 4th prince for his own good after he ascends the throne (forgot the exact reason behind her motivation to do so) and made a fake marriage with 14th prince,,, In the 14th prince's house, she gave birth to a cute girl who's 4th prince's bt she hides the fact that it's the offspring of the emperor (4th prince) and the emperor thinks the kid is 14th prince's.. Eventually Hae Soo dies a premature death bcz of weakness in that ancient historical period and wakes up from her coma in the present world.. and we see in the last scene that the emperor (4th prince) promises that he'll chase after her no matter where she is!
You know what? They can do away with the incest part and the story would just turn out fine