Even if Mom and sis were in a relationship, MC would be joining them. They are allowing him to join their relationship. Totally not NTR.
I've complained about NTR at times, but I don't see how the MC stumbling onto an existing 'relationship' is NTR. They were doing this before he showed up, he's not involved with either ATM and it's not just happening for the sexy time lulz.
I think you are confusing NTR with Cheating. For NTR, it doesn't matter who is in a relationship with whom. In general, what matters is that a connection (which the MC either had, or on some level desired) is (or is found to have already been) usurped by another character in a sexual manner. The MC is displaced, and the object of his desire (of whatever kind) is taken from him.
Complicating this is the fact that for games, the
MC's perspective is much less important than the
player's. So in this case, you have two characters that are clearly built up as love interests for the player, suddenly being revealed to already have been taken. And yes, exactly, you are only joining
their relationship, meaning that you become just an add-on; your role as central character, as harem leader, as the primary love interest for two of the love interests in this erotic VN, has been usurped.
QED NTR*
That said, I know it's an incredibly subjective topic, but I hope this post helped to shed some light on another perspective.
*However, it's a rather weak NTR, meaning that not only will it alienate strict NTR-haters, it won't even do much for a lot of NTR-lovers.