Wouldn't seeing your sister in sexual situations with others be beyond the border and firmly in ntr (
ntr: Designed to cause jealousy by having the romantic interest involved with someone other than the MC.) territory? She's a love interest involved with someone other than MC.
I’d say it’s about the difference between NTR and swinging/polyamourous/open relationships. If everyone has sex with everyone, is it a world of NTR?
In the quote you are also ignoring the "designed to cause jealousy" part of the definition.
The sister isn’t sucking stranger’s dicks to cause jealously, she is doing it because she likes it. Likewise, the MC is not jealous of it. Same with the MomxSis scenes or the implied JanexEffi content. It’s the opposite, rather. I’m fairly sure Effi mentioning that Jane and John both taste the same is meant to be sexy.
It gets further into NTR territory with Hitomi’s scene, but even there, I’m not sure the scene is supposed to evoke jealousy. Hitomi is not a girlfriend. She’s just a random sexual encounter. At most she is a sexfriend. As far as I recall it is also less of a jealous "That’s
my girl!" reaction and more of a extreme reaction to the unexpected situation. ("She deserves better!" is how the MC phrases if I recall correctly.)
I’d personally categorize the scene with the mother as NTR, because the following dialogue has some strong themes of humiliation. It’s a sex scene intentionally made to make the players feel bad and the MC feels betrayed. But even then, it is a bit ridiculous to call a wife having sex with her husband NTR. If anything, you, the MC, are cuckolding the father.
At least for me, the formula goes like this:
1) The Cuck has to feel entitled to the Stolen Lover. (Cannot just be a random person.)
2) The scenario has to feel like a betrayal of their relationship. (Cannot just find it hot and approve or be indifferent.)
It’s all about expectations. If you are the 20th person who fucks the town slut, it’s not NTR if there is a number 21. If you go to a prostitute, that’s also not NTR. If you go wife-swapping, that’s also not NTR. If you randomly fuck someone at the nude beach, it’s not NTR if that person then also fucks someone else. If you and your sister both have fun at the glory hole and then also (later) have sex with each other and each get another sex buddy, that’s not NTR. It’s just people having sex.
Of course that’s just my interpretation. It’s controversial enough that I decided to include examples in the FAQ instead of saying it definitely is or isn’t NTR.