Netorare has changed so much over the short years. Back then it would be strictly for couples or married ppl, but nowadays in porn, especially in hentai. The feeling of being taken away just has to be there, for example, you can have a Mom/Son netorare or Brother/Sister or even Childhood Friends NTR, they're not in a relationship but if the other party (usually the boy) likes the one being taken away (usually the girl) then it will be tagged as NTR.
I think games easily trip this tag too because most porn players see every girl they can romance as "their property" so if one goes after another character to romance, some players see it as "being taken away" from them and therefore they feel NTR despite not pursuing the girl.
It was never just couples and married people. Huge part of NTR from the Japanese is just that, sisters, childhood friends, etc, as you said. And yes, it does limit some storytelling potential, with multiple routes and the like, and part of the reason so few stories have more than say 1 male friend while having like 30 female ones. If you had a game with an even number of guys and girls, and it was say about, how the choices you made affected who hooked up with who over a weekend... it would still get tagged as NTR, even if shaming the player was never the intent.
I will say that early games in the porn space had stuff like this a little more often, and it didn't seem to be as hated years ago as it seems to be now either. IN older games it was usually an alternate, or BAD route that you would get if you fucked up, like cheated on your GF, and bad things would happen, or didn't get enough points or do the right things. This game falls under that, AND it give you the option to say that what Chloe says is bullshit. That and the final NTR scene is the hardest one to get in the game anyway.
Then again, it can be a serious turn off to some people, especially those who identify strongly with MC. And no one wants to walk into it blindly. It's called the porn equivalent of horror for a reason.
With this game though, I see the NTR as similar to Power Vacuum, humorous, and completely avoidable. I have trouble imagining people taking this game too seriously, everyone in the series is an awful awful person, or just a pure idiot. Everyone is fucking everyone else, everyone is constantly cheating on each other and lying. It's absurd. But the tag definitely does fit here, the entire point of the game is to fuck Amy, something players have been trying to do since what, Chloe 2? And if you mess up badly enough she loses it to a cartoon rich prick.