The "NTR-ness" of this game was hotly (and nastily) debated early in its life, and I'm told that the tag ended up getting added just so that the "oh, no, nothing that even vaguely resembles NTR for me" crowd had fair warning that they might want to stay away.
What we've seen so far wrt people in relationships with the MC:
- The MC's mother being coerced into sex by a cop. Also, the mother appears to be having an affair with a fellow teacher which the MC is sort of investigating. However, probably doesn't count for much, because the MC doesn't appear to have any "more than normal" attachment to his mother.
- The MC's sister having sex with her boyfriend and then with the MC's best friend. Again, nothing here, because there's nothing going on between the MC and his sister.
- The MC's girlfriend stripping in front of the MC's best friend and having him take "Playboy-style" photos of her. So, small element here, perhaps.
But, that's as much as we have so far in this vicinity. Thus, if it qualifies as either, it's pretty mild.
IMHO, the early raging about "you didn't tell me this had NTR in it" was probably the result of #1, with people jumping to the (unwarranted) conclusion that the MC and his mother were going to be... "closer." Because every story has the MC going after his mother, right? (Well, not this one.)
Hope that answers what you wanted to know.