What
Enlight432 said, the NTR in this game is only possible if you play the MC as a "dumbass" that is, you have to purposefully piss off & alienate Sylvie for her to get cozy with Henry.
To me, true NTR is about the MC "doing his best", but it just pales in comparison to what other men can offer - and the NTR men cleverly exploit that weakness in your relationship to seduce your gf/wife.
A bit of a shame to see the NTR play out the "dumb" way in this game, but nonetheless, this is one of the higher-quality games I've had the pleasure of playing, so here's to a successful project!
I just hope you give Iris the opportunity to choose Henry over MC, but without making the MC act dense to achieve it.