I'm not privy to the Dev thoughts, but from all indications so far, there will be NO incest, not even with the adoptive (surrogate) mother.
That doesn't mean that BD can't change his mind, I truly, truly doubt it. Why? Patreon is his job. That's his moneymaker. He will not do anything to jeopardize that, and with the rules imposed on Patreon by the payment processors, that Patreon has to enforce under penalty of litigation and financial troubles, incest is NOT going to happen. Unless those rules change (wouldn't bet on it).
I know that you are all well informed and all that, but this is way bigger than just making a few scenes with this character or another. For him to please all of us that enjoy that content, he would have to drop all income from Patreon, without any real payoff for doing it. Just go to his patreon page and see how much we are talking about here, and ask yourselves if you'd do it, I don't need to think I wouldn't even consider it.
Peace