You're probably right, but even without talking about incest. It seems pretty absurd to me that you can't, for example, my wife lives with me, but you can't establish relations between her and her family who live in another house, so this family I created with the same last name is just a random family with no connection to my wife and me... I saw that there is an icst patch in the thread, and you can very well have family members (provided they live in the same household as you) with a family connection and a fuck buddy at the same time, but that's not even the point. But I know the game is still young in its development, so we can hope for improvements on these different aspects.
Oh... And I have a question if you don't mind to answer. I know that we can take control of the NPC belonging to the same household as the MC, but is there a way to take control of another NPC?