I must admit it feels like there isn´t really any NTR in this game, mainly because everyone appears to already be sleeping with everyone else. Possessing the mom and sleeping with the neighbors pretty much tells you that they are already sleeping with each other. Your relationship with the neighbor girl is open/FWB and MC and his sister have already banged before the story even started. And it is way to easy to convince the neighbor girls dad to sleep with her, if you posses them.
So it really just feels like you have waded up in a giant swinger party, where everyone is already sleeping with eachother. There isn´t really any relationships to NTR in the first place. I actually question the necessity of the magic in the first place, because I am pretty sure all the people involved would be down to just having a giant incestuous polyamorous orgy/relationship if they just talked it out with each other.
Late as all hell reply, but that's because the developer's idea for what the this game is tone-wise has evolved a lot over time.
This game started development back in 2017, and during those days, it was, at least to my knowledge, taking place in an ordinary slice of suburbia where the established relationships were treated as they normally were and monogamy was a thing; the core idea was that, with the book and the succubus's influence, that the MC would totally upend this and start corrupting the place and everyone there.
Somewhere along the way, however, the whole neighborhood was morphed into essentially one big polycule, and as the result the Book itself lost its whole "corruption" angle, and turned it into some spices for an otherwise already horny community; honestly, with the characters being the way they are, if the MC just, like, told them about the Book and what it could do, they'd probably be down for at least a decent chunk of it.
My guess for why this is came to pass: the developer probably realized that those sort of really messy social scenarios that would be caused by some kid becoming a total degenerate and using magic to fuck his mom and his sister and his sister's boyfriend and his sister's boyfriend's sister would be an exposition black hole to try and explain why the MC could lay pipe (or have pipe laid upon him) across his family and neighbors with no lasting ramifications, be they positive or negative... even though the Book has a whole antimemetic component that's critical for its sorcery to function already, but I digress.
While the game still has edge in places (and IMO is kind of an awkward contrast with how whimsical 90% of the rest of the game is with all the fun sex scenes and what-not), it's ultimately a consequence of the developer spending less time on "why" consequences don't find you and more just trying to crank out enjoyable scenes. Whether or not that realignment was one for the better is up to personal discretion.