Hardly any storyline. It's just a brother and sister having sex. One episode and they've had sex twice, already. I started watching the second episode of this series, and where I left off - they were about to "do it" again. Unless you like flat-out hentai, avoid this. If you haven't yet, just watch the prequel - it has more of a story than Yume no Naka, although barely.
I might have liked it more if there was more to the relationship than Sora lusting after his hot older sister with big breasts, because that's really all it is.
The only redeeming quality are the seiyuus, and I honestly feel bad for them - to be associated with an OVA like this. For the content of this OVA, their acting was quite good - the art wasn't too bad either, I suppose.
I can tolerate incest. I can stomach watching hentai. But if you're gonna put them together, at least come up with a storyline.
Don't bother with the manga, either. It's just more hentai scenes with more people.