Aki-Sora isn't your run-of-the-mill brother-sister incest story. There have been too many OVAs to count where a boy and his step-sister(s) of no blood relation at all, made quite clear early in the story, have expressed their forbidden love. One of the attractive points of Aki-Sora, though, is that Aki and Sora are, in fact, full siblings. Finally, the 'I' in incest was dotted, and the 'T' in taboo was crossed, like the line that Aki and Sora cross when they give in to their desires for each other.
A point I find particularly appealing about Aki-Sora is Sora's twin sister, Nami, whom he is not incestuously involved with. Had I been writing this story, the temptation to have the twins become involved would have been too great to ignore. Twins tend to be closer to their other "half" than to anyone else, so it seems reasonable in an anime about sibling love for there to be strong feelings between the two.Yet Aki-Sora doesn't rely on cliche here, and instead presents us with a more original story: one where, instead, Sora's twin is just the opposite, and there's no particular closeness between the two. Beyond that, there's an amazing love polygon that will surely make you take notice.
Backstory is adequate for understanding the relationship between Aki and Sora, but not so much that we lose sight of the main action, which is well paced. I would like to have seen more backstory with Nami, however. The characters are not completely cliche despite the mold this anime fits into. Sora especially is a bit more dynamic than we might expect. He does, however, blush far too easily. One interesting thing to note is that the relationship between Aki and Sora seems to be a bit of an Oedipal complex, with Aki acting as a stand-in mother. Their mother is not absent (as we see in the sequel), but is rarely home.
I rate the art higher than I might have otherwise, only because more than once did I find myself pausing to admire it and to take screencaps -- but not for the reason you think. Sora is beautiful [I am fonder of boys than girls], and in many scenes, the background and landscapes have an almost magical quality thanks to the shading and glow on them.
The female voices were good, but the seiyuu playing Sora (Kinoshita Sayaka) was quite good. Some may think that Sora sounds TOO much like a girl, but even though there may be a few moments when he sounds unconvincing, it works for him. As Aki tells him after she dresses him in a girl's tennis outfit, "Don't worry. You look like a girl anyway." The only time it may be truly distracting is during the sex scenes.
This was a fun one to watch. I'm more than a bit hentai myself, which is a requirement for enjoying it. If you're not, skip it. There's no point in wasting your time. Thankfully, though, it seems to blaze a new trail right beside the beaten path, so even though some notions may seem worn out, old, and trite, others will be refreshing, eye-opening, and enjoyable.
I followed up this OVA with episode 1 of Aki-Sora: Yume no Naka, which I can say only improves on this one.