Considering his previous post about turning slaves into rations, I'm not sure about that
Don't see the problem if it will be an option that you choose before starting the game. I'm pretty sure that people would have requested it anyway at some point. The game can be pretty dark and games with incest are usually going for light-hearted romance route, or the devs goes for the least difficult way and making them all perverts. Here in the other hand, it might be very kinky, because MC actually will turn them in actual slaves. I imagine two approaches to this: 1)MC and his family running business together, they more like npc's with whom you might interact. They can give you quests, help you with training and sometimes just help you with something you ask (money, weapons, information). More or less it's somewhat justified light approach we usually see 2) Your family seriously fucked up something and they in danger. The MC can save them, but the end price for them will be no better. You are a real tyrant, you doubt anything they did so far (justified or not). Slowly you seed thoughts in their heads that their family will be much better in your hands, if you be in control of everything. Eventually, after you break them enough, as a final power establishment, you suggest them to sign the slave contract. That's the evil path.