Hey dev.
Is ntr only to granny? (I hope)
Did mom and other got ntred??
It depends on your choices, and what you consider NTR; that is, there are things that do not happen in reality, they happen in another dimension within the Veil of Shadows (this is explained much better in Love MorteM).
Another is that there is no humiliation, because it is not an alpha/beta approach; and even the MC, already has his relationship, and accesses incest relationships through the Coven/witchcraft/traditions; are concepts that break with the traditional NTR (here it has been baptized "PseudoNTR").
There is a route that would be the most similar to the Harem, avoiding several things in the story.
But it loses grace, that is, the adrenaline of the consequence or reward of the choice is lost.
It's more debauchery; at no time are you "robbed"; if you want to try the "softer" (and I reiterate it's not Harem!!) try the weight choices; go with Jessica & Leona (consequently, don't go to the beach with your grandmother and Adrian); don't go spying on Roxy (very important), and in 0.3 (coming up in a few weeks), be frank with your girlfriend and cut off the relationship.
That is the "safest" way (within the standards of the VN) for now in TFTS; but the grace of a VN like this, is to think like Bible Black; without having the weight of the NTR; and choose by taste or curiosity.
Greetings!