What relationship lmao. So if your gf fucks someone she used to hook up with back in college, it is not cheating right? Because their "relationship" is much older lmaoo.
The mental gymnastics are hilarious here. Almost as hilarious as the dude that claims to not care about facepalms but brings them up everytime he gets one lmaoo. And Ive seen him calling people insecure, joke writes itself.
If your MC is dating the sister she will fuck another woman(who mc is also fooling around with) behind his back. If you dont consider lesbian sex ntr, then it might not bother you, but it is still ntr. It is avoidable now though, I think: You can make the decision for the sister to cheat or not to cheat.