It’s a "everyone has sex with or lusts after various people" game, though most of it is in the lore instead of directly shown. Your sister mentions having had sex with four people at once and you can find her at the glory hole. Your Mom/Son incest route is mirrored by a Mom/Daughter route you can get in on at some point. Your first sexual encounter (Effi) is basically the town slut… then there is the sex world, where everyone has sex with everyone, though you will only see one character actually going at it.
Walking in on the parents is the closest to the humiliation type of NTR the game gets. Afterwards, the only family sex scenes are between the two girls and the MC.
Basically, if you cannot deal with your sexual interests also having other sexual interests, this game might not be for you. It’s never downright NTR (though the sister calls the MC a cuck a few times), but it comes somewhat close several time.