correction, he raped someone, also, its implied that he is gangbanging the sister. and ntr is not just what happens, but what is implied. as for "not someone the mc is in a relationship with" is also a question, since its someone that might be.Some people just don't know what NTR is, Darnell fucked someone in the toilets you don't know who, and it definately wasn't anyone the MC was in a relationship with.
in porn games, if someone is fucking someone, and the mc is not one of them, its fair to consider it ntr. because ntr is a massive tag that can mean anything from 2 people fucking, to someone fucking a love interest. and since the forum has bundled all of them in the same tag, then yeah, it is ntr according to the forum own definition.