The age itself is not the main issue imo. While I'm not a fan of that genre myself, it is explainable. There are reports of instances or even memes where the girls look like being old enough, that's why the term jailbait exists. My main issue is that the little sister clearly looks younger than it should be. Physically and mentally.
It is normal for some feel attracted for girls who are nearly 18, after all it is just a matter of numbers (ever wondered why so many pornstars are around 20?). We don't do it not only because of legality, but also you aren't aware if mentality she's also decently grown up, and fortunely not all of us are scumbags. Some are, some aren't, and even some over 18 aren't. That's why the older sister doesn't bother me at all. However with the little sister it's too much, even her expressions, reactions and events (like needing someone to take her to bed) force her too much to the kid stereotype, and not to the teen one. It isn't only the body, but everything. How she looks, how she acts, the events, how others act to her, etc... You can't even try to put her on the "area" in which a girl is developing to reach adulthood, I can't even even put her above 12... which is understandable why only those with that cliché will like her (I'm not trying to shit on those who like girls like her btw, I'm just saying I don't).
I do agree with you the game is well written. But personally it woud be nice to have something like a timeskip in game, as I mentioned in a previous comment. At least to a point she at least doesn't act like a child. I'll probably still play the next releases, but I'll most surely skip her scenes. It is also a thing that puts me behind when it comes to support, and maybe to others, because I do believe the quality in writing deserves more supporters... and maybe don't as well due the same reasons. I don't want to complain about the dev, and as I already mentioned I believe the writing deserves more support, but I understand as well some instances as to why some feel reluctant to support.
P.S.: I still believe Viki and Juliana give huge vibes of cousin and aunt btw. I know they aren't, but the vibe is there, is a lost oportunity.
Sorry as well for the wall of text, just didn't want to misinterpreted.