There is a scene where Eve fuck with another guy. This is the only scene like this that I have planned. It is made to add some background to the story. It’s avoidable and not very relevant to the plot, as the MC will not see or know about this event. Does this count as NTR?
short answer, if she is a li, then you may have a shitstorm in your hands and people fighting about it being ntr or not.
now, if I may. I will just state my opinion which is based on my experience with similar situations in this site.
IF Eve is a LI ( short for Love Interest) and since she is supposedly a family member, the incest players will avoid her like the plague. you risk losing a pretty big chunk of the player base for absolutely no reason at all.
If she is not a li and will NEVER BE ONE, then the scene on its self is pointless since you yourself make it avoidable with little relevannce to the plot.
in my humble opinion, you would do far better on not having that scene at all and not ruin a li ( or possible LI) in such a way ot losing on your subscriber pool.
also, I would like to point out something that you may want to keep in mind. you say Avoidable but from the context of what you said, you propably mean Skippable.
Avoidable means that the player gets to stop it and never happens which is Universaly acceptable by the players.
Skippable is when the player gets the option to Not see the scene but the scene stil happens plotwise. which is universaly considered POINTLESS.
I could offer plenty of examples with games that tried what you plan, and players didnt like it. for refference I will just give the name of one which no longer goes by that name because the dev had a li, have sex with an npc. the game was Red Falls
If you want the game's new name and a peek at what happened in that thread, I can give it to you in a pm. if you ned any info on how this kind of plans go, I could also point you to several games that did that and how they fared.
best of health.
EDIT: I hope I am completely wrong in this but there are too many flags for my mind not to go there, mostly because it is not the first time I have seen this. The fact that it is a single scene of a li with another man, the fact that it is a family member (and from what it seems a main female character) and the fact that it has very little to do with the plot, makes me think that this was a request from a player who wants to see this short of thing.
just leaving this here, since as I said, I have seen this thing happen more than once.