If you take the incest out for example. Deb is a women who is nearly in her forties would you be okay with your teenage daughter fucking a woman in her forties. Keep in mind your teenage daughter has been emotionally abused and lied to by her mother for years and is still struggling to deal with her death. I don't know about you but I wouldn't want someone like Deb anywhere near my daughter.
Lets not forget this 40 plus year old dyke that's actively seducing your teenage daughter, is also responsible for keeping you away from her and fostered her and your ex wife's hatred for you behind your back.
Of course the MC doesn't know about this yet, but when he does, I think that will be the turning point where Deb is removed from the picture and looses her harem.
I understand wanting to think happy thoughts, and the world is full of rainbows and fluffy white bunnies, but this isn't one of those stories. Frankly, it's more likely that the Dev is trolling everyone with Deb, and has lost track of what he was doing with the story in the process.
Edit: Think on this for a minute. If Deb is so wonderful, and well meaning...why did his ex wife reach out to the MC, and not Deb. If Deb is such a nice fit for his daughter, and every woman in the household, and capable of protecting and caring for them, why wasn't it her that the ex wife reached out to. Something is off with Deb, and I'm hoping it'll come to light soon.