I agree, I don't really think this qualifies as NTR either since the reason the main character finds her sexually appealing (and also repulsive) in the first place is her blatant promiscuity. Later down the road when/if it develops into a romantic interest, and she still keeps fucking around, slap that tag on it - it'd be full blown NTR.
Then again, I also recognize that for people who have a particular aversion to NTR/sharing, that's a meaningless distinction. On the flipside, if you are into full blown jelousy-laden NTR, you'd find it mistagged if it did have one, so uh...
To keep it on topic, I really like the dynamic
Lemon Mint has crafted here. The push and pull between the two sisters is interesting and sticking a straight-laced protagonist in the middle of them makes for some good drama. Really looking forward to seeing how it unfolds.