Man, calm down since it is clear that you got intenzo for a thing of nothing, NTR is when a person you love is stolen from you, be it a relative, partner, etc, Netori is when you steal that person of their loved ones. But in the scene that appears at the beginning of the game, the MC doesn't love his stepmom, it seems that he barely sees her as a stranger.
That scene is more a cheating to the mc's father than a NTR, of course assuming that the brother does not appear more in the island and now if the thing becomes spicy, but if you got to play Bad intentions (there the brother of the mc is the protragonist) you will know that it is only a wink to the game that was inspired Island