The series had a pretty good premise. A pair of close siblings become even closer, too close for some. What could have been was a story that explored the taboo nature of their relationship. Whether it condemned or condoned it didn't matter. It could have even kept all of the sex.
But what we got was a story that got smuttier and smuttier every chapter with little regard to consequences.
Aki shows little regard for how her relationship with her brother could be perceived, or cared whether it was right or wrong. But Sora was far worse. Infidelity may as well have been his middle name because when he wasn't having sex with his older sister, he was having sex with some other girl.
Granted some of the events weren't necessarily avoidable. For instance, at one point he gets knocked out at a party and repeatedly date raped. He tells Aki about absolutely none of his sex encounters.
He shows absolutely no remorse for his actions. The story could have been trying to portray polyamory in a positive light, but it absolutely failed. The only time the story actually judges any of the character's actions, it judges the incest. Honestly, the incest is the least of it all. Aki leaves him out of a newfound guilt, and Sora is crushed.
Well, not so crushed that he won't have sex with the next girl that asks. But crushed. That is until he realizes what is truly important to him. And then promptly having sex with the next girl that asks. Anyway, the story ends on an incredibly unsatisfying note.
Sora thought he would get Aki back, because he really loved her and she really loved him. He was half right, or maybe not. There can be sad endings. There can be depressing endings. But this was bad. I think if the story was more focused on the actual romance and taboos of it all it would have been a lot better. A short story about a pair of siblings romantically loving each other before coming to terms with tragic taboos would have been much better. Instead the story went nowhere for too long and wrapped up in some of the worst and contradicting ways it could. Even a nice boat ending would have been better.
For a better portrayal that looks at the issue seriously, I would recommend Koi Kaze. The story is much more taboo, being about a pair of siblings with an age gap of 12 years. It doesn't ignore the taboos.
I would also recommend the similarly named Yosuga no Sora series. All three of the versions are quite different, the VN being a hentai, the anime being a borderline H series, and the manga being much more tame (it was published in a shounen magazine even).