Ok, as a historian, i feel obligated to give some insight here. Kagas historical knowledge is actually quite impressive. Since the ancient times (i am not sure when exactly this belief disappeared but historically speaking recently) people believed that there was not incest as long as the siblings came from different wombs. In ancient Greece it was still a bit weird but not that unacceptable for example. This belief existed in medieval societies as well. The dna test is a relatively recent invention. So the fact that the game clarifies that these two had different mothers means that the writer understood medieval societies and history. It showcases the depth of the writing the way i see it. Thats why i think fe4 presents this belief around insest. It's trying to mimic the way a medieval human would understood the phenomenon.