Suggestions:
1. Don't force incest, just leave it optional. Make sure people can level up their perversion other ways and make sure those other ways are just as accessible. In fact the mark of the best sex games is, ironically enough, that they never force any kind of kink or fetish or even sex partners onto the players while still leaving them all open as options.
2. In the long run, I propose having Fallout2 style multiple-stage endings. Meaning a part where each girl's future life after the story ends is talked about, with different possible endings for each girl. That way if you can give waifu endings for people who just want one girl over the others, ignore some girls altogether, or try for a harem thing, it's all possible. Maybe not everyone wants to marry the princess, after all. You've packed this game with lots of desirable women - please let players choose which one we stick with at the end.
3. And once you have that factored in get rid of the automatic forced-sterility spell at the start. Make it into a spell that the player has to cast on themselves every day if they want to avoid pregnancies. And if they don't, well, that happy ending might not be so happy when those other baby-mommas come in and crash your wedding plans. AKA the "Good ending that's not really so good."
4. Putting this all together, each girl would have a multiple endings attached to herself, and which one is pulled out for the ending montage depends on the player's actions, as would the final ending for the MC himself. Who gets the wedding ring at the end, or does nobody get it and the MC stays a swinging bachelor to the end? Which girl gets ignored completely? Which one is mad over being used and rejected later, or was the player smooth enough not to break her heart? Did he accidentally get Samarra pregnant, reject her, and then get stabbed in the throat afterward? Does the fairy king come looking for revenge or does his wife just leave him for the MC or both? Did the player gain enough affection from Samarra without being a jerk that she fends off the vengeful fairy king and saves the MC's life? Does the MC try to "convert" the lesbian tailor via magic or does he help her eventually find true love with some other nice lady?
See, the possibilities here are pretty much endless and really wouldn't require that much work - just a single picture per possible outcome and some text. Then just slideshow them together at the end.
