Not sure why you are unwilling to do a full harem route? Some of the things I was reading makes it seem like this is a post-apocalypse, is that correct? That's a genre that meshes with harem better than many others. A depleted male population and literally fighting for survival and resources is often the primary cause of women being willing to revert to sharing a man for protection and provision. It sounds from this like the MC has a choice of abandoning his mother (and a few other characters. possible a sister to be introduced?) in order to get a larger "harem" of other girls. An MC who would abandon his mother isn't the kind of MC I'd be interested in playing even in a non-AVN game, but also, I don't see why he can't save everyone. It sounds like you are giving an option of "abandon your mother and family," or "abandon a group of other women," to their fates. Basically, it feels like there is a disconnect between who you are targeting as your audience (incest-harem, non-NTR, fans) and then the game as it is being set up. Whatever you decide with the game I wish you the best of luck and I hope you find your niche but presenting your presumed target audience (incest-harem fans) a forced choice between the incest and the harem doesn't seem like the wisest move. But again, best of luck and of course make the game you would ultimately want to play and have fun playing. Because otherwise you might lose your passion, and no one wants that.
Edit: I do want to be clear that I’m relying on hearsay and speculation so if my information is flawed then that would be good to know.