NTR doesn't need to involve someone the MC is involved with or romantically interested in. The idea of a relative being stolen doesn't necessarily need to come from a perspective of the MC being sexually attracted to them. It could just be that they love their relative in a normal way, but someone comes in and "corrupts" their mind so much that the relative stops loving the MC because all they care about is having sex with the antagonist. Like, a mother could start neglecting taking care of the MC, or a sister could stop spending time with the MC (or protecting him). That's a common theme in the NTR trope.
In this case, yes, the MC does have an unhealthy attraction toward his sister, so it follows the typical stolen love interest NTR trope. But I am curious how this story is going to turn out. Unlike many NTR stories (too many, imo), the MC isn't an unpopular loser. He is very popular, and even though he's not well-endowed, the few sexual interactions some of the girls have had with him have been positive. The first negative interaction actually came in this last update, from an unlikely source, and that could lead to some soul-crushing NTR events in the future.
But yea, I'm not quite sure what to expect. I have a feeling it's not going to be a "happy" ending, but it's not gonna be soul crushing ending either. I'm leaning toward it being a "real" ending. Some heartbreak happens, but the MC eventually grows from it and moves on. That's just my theory, though. I could be way off.
