No, it is not, but judging by this being your only NTR related question, I guess you game caters to a vanilla Harem/Incest/Romance audience, it is better to not have that talk at all, assuming its the only instance it happens. The people that hate ntr are also people that hate hearing about LI, potential LI, or family members' sexual experiences, while not being ntr they have the exact same narrative purpose and evoke the same type of emotion out of the player.
That doesn't mean you should not write LI that are already in a relationship at the start of the game, it just means people don't want to know sexual details of that relationship, think of this concept something like 'out of sight, out of mind'.
Being a DIK is a masterclass in this, a majority of the secondary characters are sex workers, yet most are fan favorite characters and rarely will you see people complain about NTR. Everyone know that they are sluts, but the game never rubs it in the player's face, we don't catch them in the act, don't see photos and videos of it, we don't hear them fondly remember how many dicks they sucked and what gangbang sluts they were in the past, its all about the type of atmosphere your game projects.
This is something a lot of devs fail to get when they add these 'technically not ntr' type of content and then are surprised they get backlash, it has the same narrative purpose, doesn't matter how you define it(which trust me, people love to get stuck on their definitions). You can write a slutty girl without making her as undesirable as possible as a romance option.
Good luck with your game!