I think this manga would work better as a hentai. I'm willing to except really bizarre, nonsensical, totally not believable actions and discissions, if it leads to graphic lesbian (and/or futa) sex. But when the sex gets toned down to just ecchi, and the author tries to flesh out more of the character's feelings and motivations, the whole thing falls apart.
Part of the appeal of incest in fiction, is that it's taboo. The siblings are too in love, or too horny for each other; and must fight against and overcome their family and friends' rejections. Making the 2nd sister totally okay with incest, and wanting a piece of the pie, would be hot in a hentai. But if you think about it for more than a couple of seconds, it's retarded. I feel like the story is moving in the direction of more and more of the characters being okay with incest; except for Chiaki. That's not enough push back for such a huge taboo. It trivializes it; which makes the story boring.
At the same time, we get all of this debate and self doubt on incest from Chiaki. So the author wants depth; but also wants incest to not be a big deal. Pick one or the other. The story can't have it both ways. Either incest is a big deal, or it's not. If it's not a big deal, then drop all the introspection and debate. Make it purely horny and go graphic.