I honestly feel like this thread is a case study in "what happens if a dev even mentions NTR in any context other than black or white terms." Instead of either coming down hard on one side or the other, and saying "yes NTR/S will definitely happen in this harem/incest game" or "no NTR/S will definitively not happen in this harem/incest game", more of the conversation in this thread has been about NTR/S than basically anything else because no actual stance has been stated.
I get that you may not yet know if you want to do it or not, or may want to do it, but are not sure how you would actually do it, but my only advice would be that the longer you put it off while pushing the game as an incest and harem focused game, the more of a shitstorm you will find yourself in if you then add in NTR/S suddenly in a later update. ESPECIALLY if the game is still kinetic at that point, and any NTR/S is not optional/avoidable.
Because at this point, any players following and enjoying this game are almost certainly not NTR/S fans. As others have stated, harem/incest fans are very often mutually exclusive with NTR/S fans. And from what you have said, the extent of NTR/S that you would want to include (having it be a small focus, on side characters from outside of the family, etc) would almost certainly not really appeal to the devoted NTR/S fans either, who would prefer a game that is more focused on those genres, and likely would want them involving the main love interests.
So the audience that would be catered to with that combination of tags would be people who are into incest harems, but ALSO like a little bit of NTR/S, which is a fairly small subset of people. Most of the devs who have managed to pull off having both NTR/S and incest + harem in the same game, have done so with great difficulty (and few have actually managed to complete their games due to the extra work, since the only ones that have been successful have ensured everything was optional/had multiple paths) such as Power Vacuum, Taffy Tales, etc. Even the aforementioned NSFW games series abandoned any of the non-sharing paths after the first game because it was just too much work (and the dev simply wasn't enjoying it), which I respect him for doing it, even if I'm dissapointed because I have no interest in that kind of content despite absolutely loving the first game.
Anyway, that's all to say that it's really up to you if you want to add NTR/S, sharing, whatever to your game. But I just really suggest you decide sooner rather than later if and how you plan to do it. Don't worry about "spoiling" things. Just decide for sure if you want to have that kind of content in the game or not, and whether you intend to make it optional or not, and then just say something like "NTR/S will not be added to the game, ever" or "NTR/S will be in the game at a future point and it will be *optional/avoidable* or *not optional*". Then people will know if the game will be for them going forward, or not, and be able to decide if they want to commit to following the game, and hopefully consider supporting you as well.
