Im just posting what happened in my own playthrough.
I got the elder sister to the point where she finds out the boyfriend cheating and she breaks up with him. However, after the bikinishopping I never got any further interactions with her, and the relationship status never changed from "no". While continueing the game and establishing a relationship with the other two sisters, it ended up with the elder sister doing the "confessions" while talking about her upcoming wedding (which she shouldnt have) with Brody (who was long gone by this point). During her confession she flatout tells the guy that both sisters are pregnant and theres only the brother in the picture. Then she goes "no, that couldnt be" and they have a talk about the situation. Storywise, it shouldnt make sense.
The point system, its possible that didnt do anything. Considering how its a fairly "do or dont" game (which is the case with most erotic graphic novels) I was a bit confused by it. First I thought it meant corruption = game over, but then I realised the more kinky stuff you did the higher it got, so I felt that COULD have been related. However, it never really did much with any of the decisions, so I suppose you are right on it having no effect.
As for Alina and Ana, again it's just my findings. During my first two playthroughs where I did sleep with ana and alina I never got the james confession (yet near the end he did mention they're having sex together, even if both girls are in a "relationship" with you). During the third playthrough I did NOT enter a relationship with either, and I got the James confession.
I imagine this game's story looking like a branch, and for some reason scenes that should happen, dont happen, and scenes that shouldnt happen (or even be possible) do happen. Its like, if certain events did NOT happen (like getting together with the elder sister) the game assumes you never did anything with her and she's still in her relationship. Its hard to put into words, English isn't my main language, I hope that makes some sense.
Anyway, I am kind of sad this game will only have one more update as Im a sucker for games that establish relationships (even if its porn). I think this game reached a solid 2nd place for me, right below Katawa Shouju (which is a great emotional rollercoaster) and just above the Sagara Family (which I just love for the sometimes silly writing).