there's no ntr - just the tags over here are weird and applied by rolling dice.
There is a scene where Your Sister/Friend and Vicky, which are 2 potentional love interests, spent a night together (avoidable).
By definition it's NOT NTR, as no one of them wants or tries to steal MC's love interest away (which is crucial for NTR).
So if you call this build-up into a 3-some relationship in the end as "ntr", so it may be called ntr, as MC wasn't present
PS: that scene only happened if you are on BOTH paths (as said, it's the build up for a possible 3-some-ending).
Normal people would have noticed that anyways, and even the update later today will hint it.
I just wonder why other games with pure lesbian content and male MC don't get NTR tag applied, but hey, it's just a tag. If people feel better that way, so why not?