I just started playing the game and reached the point where Alessia has Gwen going down on Chloe. You are right, this is textbook NTR, at least to this site's definition of the tag:
Designed to cause jealousy by having the romantic interest involved with someone other than the MC. Gwen is MC's love interest, Alessia is an evil bitch as far as I know and care and Gwen is involved with someone other than MC. *looks at definition* *looks at game* Yep, NTR.
I for one (and the definition) don't care whether it's by men or women.
Also I think the "Will you romance your own twin sister?" from the OP is rather misleading. I had no idea the game was about mind control when I downloaded it and am a bit disappointed now. Dominating (Alessia's words) via mind control is not romancing. I was hoping for a love route when I read "romancing".
And "whose older identical twin sisters help him to lose his virginity"? They don't help him, they put him up to it, they manipulate him.
The whole intro text sounded much more consensual than I now realize the game is.