OK, I think I'm about ready to discuss something I've been thinking about for a while. I've been giving it quite a lot of thought whenever I play the game. It's not a gameplay issue, fuck no, the gameplay is beyond irredeemable at this point so never mind it (although the gameplay is part of it I guess), it's what I feel is a big omission in the story and a huge missed opportunity. And hell, maybe it's even been addressed in more recent versions, those patch notes are incredibly long and I might have overlooked it since I don't keep up to date with the latest version, but I strongly suspect it hasn't been. Anyway, please read the ENTIRE post to get my whole point, don't just dash off to write a "no, you're completely wrong and don't know what you're talking about!" angry rebuttal before you see what I'm actually getting at.
So let's talk about Eve. More specifically, let's talk about Eve's character arc. Even more specifically still, let's talk about the character arc that Eve doesn't have, even though she absolutely should and there are even hints of it already in the story.
First things first, Eve is a child. No, stop, don't argue. Yes, I'm well aware she's actually 18 temporal years old at the start of the story, but her character is still that of a child. She looks like a child (tiny, flat-chested, hairless), she dresses like a child (one-piece dress, no bra, hair in girly pigtails), she acts like a child (playing hide and seek, skipping around singing nursery rhymes) and everyone treats her like a child (you hire a bouncy castle and a performing clown for her birthday party). Your uncle's intention was blatantly obviously to keep her as a child for as long as he could (for his perverted desires). OK? Let's not argue about technicalities: Eve is a child character. She is written as a child. Agreed? Cool.
So here's the thing: since Eve is a child, you shouldn't normally be able to be sexually attracted to her, unless your character is actually a pedophile (or at most an ephebophile). You can like her as a person. You can even fall in love with her, as a person. But the average person simply isn't sexually attracted to children, the same way the average man wouldn't find Katka Kyptova sexually attractive unless they have a thing for huge muscular bodybuilder women, or the way a straight man wouldn't find the most handsome man alive attractive; the physical attraction simply isn't there.
As a result, there should only be 3 circumstances under which you'd be able to have sex with Eve:
1) You actually are a pedophile and find her sexually attractive.
2) You're doing an evil bastard playthrough where you pick up where your uncle left off and don't need to find her attractive because you're only raping her to make her and Anna suffer.
or, and this is the real point I'm getting at here:
3) Because she wants you to.
Apart from her love for her sister, what is the Eve's most notable character trait? It's simple really: while she enjoys being able to be a little girl, she wants to be a woman. The signs are all there in so many of the interaction scenes you have with her: the way she skips down the main stairs until her skirt flies up to flash her panties at you then smirks at you because she knows you were looking; the way she tries to flirt with you while lounging at the poolside; the way she gets tearfully upset when you catch her putting oranges in her dress so she looks like she has large boobs and you laugh at her; and most obviously, the scene in the bath where you watch her get out and get dressed and she flat-out tells you to her face that she likes it when you look at her like a woman rather than like a child. If you cultivate a close, caring relationship with Eve, then she wants you to love her like a woman, not like a little sister.
My current playthrough has been going on for quite some time, but I've never engaged in sexual activity with Eve, apart from stealing her panties one time she she was hiding under my bed, and feeling her up one time when she was in her cubby hole, neither of which led to anything. And honestly, despite my attempts to roleplay the game properly, both of those moments were rather jarringly out of character because the protagonist shouldn't have been willing to do perverted things to a girl who, to him, looks like a 12-year old. There haven't been any events I've come across that actually lead directly to sex, other than the kitchen scene where you come in and casually tell Anna and Eve that you're horny and have sex with one of them, which is very much a "bad guy" event anyway and doesn't act as the culmination of a character arc. The only other way I can see that I can enter a sexual relationship with Eve is ordering her to come to the Master Bedroom and casually fucking her in a generic, repeatable sex scene that I could have done at any time. According to the wiki, Eve won't offer herself for any sexual actions you haven't proactively performed on her first, which I frankly think runs entirely contrary to her character (quite apart from how she wants to be see as an adult, she's also a very forthright and unreserved person who isn't afraid of saying what she wants, unlike Anna).
Because the game is at its best when the characters DO have proper arcs. The way I played it, I was as kind as possible to Anna, but as my character said during the intro, I wouldn't lay a hand on her until she wanted it. Oh, I won her over, took her out on dates, kissed her once she was obviously starting to fall for me, but I didn't have sex with her even when the game prompted me to, because there's a scene I was specifically aiming for that's a natural culmination of your blossoming romance with Anna- the one in the laundry where Eve breaks the painting of your late Aunt, and Anna is terrified that you're going to punish Eve harshly, but instead you and Eve play a prank on her that makes her laugh. That was the proper turning point in her story, the stage where she finally understands to the depths of her heart that you will NEVER hurt either of them, and she can trust you completely, and so it led to our first time together as she had completely fallen for me. It's a beautiful story, and the best thing about the game.
Eve doesn't have anything like that. And that's a great missed opportunity I think. Eve's character arc should be about her falling for you and growing frustrated with the fact that you don't see her the same way. Maybe she's happy for Anna for finding love with you, but can't help feeling jealous as well. Maybe she starts coming on to you more and more openly, to the point that she's unintentionally making you uncomfortable with being propositioned by a child. Maybe she should ask Anna to bring it up with you. Either way, it should have a proper conclusion and turning point scene: maybe it comes to a head where she openly confronts you about it and confesses her feelings to you and makes it clear to you that this is what she wants. You should have the choice of making an effort to look beyond her physical appearance (beauty is more than skin deep) to return her love, and only then be able to start a proper sexual relationship with her, after which you'd be able to "get used to it". (The natural alternative would be to tell her that you're sorry but you just can't handle it, with the obvious follow-on being to use the lab to reverse what your uncle did to her to make her grow up again, which is undoubtedly an eventually-intended function of the lab anyway).
Of course, the problem is the aforementioned mechanics issue: the game emphasises complete open world freedom, irrespective of how this segregates the story from the gameplay. There is literally nothing whatsoever stopping you from refusing to take advantage of Anna during the intro and telling her that you find her attractive but won't touch her until she wants you to, and then the instant the intro ends summoning her to the Master Bedroom again and fucking her from dawn 'til dusk right then and there. The game simply refuses to put any kinds of restrictions on your actions or tell you "you can't do that", and giving Eve a realistic character arc would run contrary to that (it also has to leave room open for other character interpretations e.g you're actually roleplaying your MC as a pedophile who couldn't wait to get all up in Eve's childlike pussy, or of course the straight-up monster route).
But I still think there needs to be a story expansion for Eve picking up on all the threads about her wanting to be a woman and leading to the proper development of a genuine and natural sexual relationship between you, rather than just "Hey Eve, come here, I want to fuck you". She's the most central character in the game after Anna, after all, she really does deserve as much content as possible, and it's such a natural story to tell (why else are there already so many teases for it?).
I hope this provoked a little thought, because I really think it'd make the game better.