@Mlemplar To be fair one of the main reasons incest is conquered wrong is the increased chance of passing on "bad" genes without enough genetic verify from the parents there are less potentially "good" genes to replace any "bad" genes increasing the likelihood of genetic defects, disorders and diseases
In this case they all shear the same farther and each of tbeir mothers come from different races they each inherented more traits from their mothers the farther literally wanted to take the best genetic traits of their mothers and add them to his royal line so in this case there is more then enough genetic variation between the parents even if they are half siblings.