That said, it's not just "hentai/porn logic." I've known a number of people in the polyamory community who embrace the sharing the love attitude.
Which is fine, nothing will ever be perfect. And I'm really not against polyamory at all, I'm just a little against it when suddenly almost every character out of ~100 turns out to be that way, even those where you wouldn't necessarily expect it. In that case I always feel like the writer chickened out because it's always the easiest solution and at that point it honestly starts to become a bit boring.
Hm dunno,feels like you are putting the relationship on a specific pedestal which in turn colors your view of Eve´s character.
She hasnt spoken out against the idea(on the contrary,Odette hints towards her being okay with it at last in the constraints of it being her sister)and its not like strict monogamity is a theme that fueled their relationship in the first place.
Well, DC always says he wants his characters to be consistent and to act within character. It's just a step too far to think that a girl/guy with serious issues like Eve, who has basically just overcome most of her/his problems, would suddenly go such a step. The question one should ask is, why would she do it? And if the only real answer ist "because it kinda solves a plot problem" then that's really not a good solution. To me it's simply a somewhat significant difference whether they have a threesome together (which is in my opinion what the game hints at) or whether she says to MC "do what you want without me". The latter is a totally different level in any relationship and I don't think it suits Eve's character (that well).
Out of curiosity,what kind of development and dialogue would you expect for this narrative turn to be warranted?
The point is more than her story is done. There won't be any substancial further development in any direction and as such no further larger dialogs. Even though I don't really agree with it, I think they could write a few additional arcs to make anything fit to some degree, but there's no indication that they'd do that. And as the story stands right now I simply think it's unfitting.
Of course all of that is just nuance, but then again, if they are planning to just add that sharing stuff without a threesome scene as a quick addendum, they just could have added it now. The fact alone that it didn't happen to me says that they don't want to go down that way, either.