I suppose it depends on the player, but the specific circumstance quoted was the MC taking away his father's wife--mother or step-mother. If the father doesn't know about the son cheating with his wife, it isn't NTR.
So, in the example you gave if the story is told exclusively from Victor's POV, cheating with Claudia and the husband not knowing about it isn't NTR. Same with Claudia's POV and her cheating with Victor or the son. NTR isn't a special tag for cheating. It's about jealousy re significant relationships. From a gaming perspective, one of the protagonists has to know about the cheating and presumably be jealous for it to be considered NTR.
I played the game and Victor is not one of the protagonists right now . . . I didn't get far, so you could have switched POV's at some point, but so far your story appears to be from the [son's/step-son's] perspective. If the son establishes an intimate relationship with his Mom/Claudia--keeping in mind that intimate doesn't have to mean sex but does imply a LOVE relationship that is not a typical familial mother-son relationships--then her sleeping with Victor [could/would by many] be considered NTR. But, even if the son doesn't LOVE his mother in an intimate way, protective instincts sometimes kick in and the son could be jealous of Victor fucking his mom. That could be considered NTR.
Good luck with the game by the way. I did download and play because it looked promising, but I got stuck on the second day and couldn't figure out how to advance the story, so I gave up.
