It's very funny, although it's mostly not meant to be. I think. Anyway, I found it hugely entertaining and I'd recommend it. It's another incest anime, by the way. The title translates as "I'm in love with my little sister", which explains why you won't see many English-language reviews using the English title.
It's based on a ten-volume girls' manga by Kotomi Aoki, who incidentally won a 2008 Shogakukan Manga Award for the twelve-volume spin-off series, Boku no Hatsukoi o Kimi ni Sasagu. So there's lots of it. There's also a live-action movie (!), plus another live-action movie based on the spin-off. Goodness me. Looking at a wikipedia summary, it looks as if this OVA is covering somewhere between a third and a half of the manga.
Yori and Iku are twins. (Not just incest, then, but twincest.) Yori's the boy and Iku's the girl. Yori's a hostile, surly, short-fused git who clearly hates himself and bites off Iku's head if she so much as squeaks, but will explode with protective rage and start beating up anyone who teases her. He's also secretly preparing to transfer to another school. Theoretically it might have been quite interesting to try to guess the causes of Yori's frustration, but in practice the show's told us everything in the title. (You could try showing this anime to your friends without telling them what it's called, although you might get some extreme reactions to your little surprise.)
Meanwhile Iku is a happy, bubbly girl without a care in the world. She loves her brother, but only in a normal way and she doesn't seem to be suppressing anything. Mind you, she does seem to think it's normal for a brother and sister in high school to share a bedroom and for her to tell her friends details of how he slept the night before.
What's interesting about the plotting, I think, is that you can tell it's adapted from a much longer story... but in a good way. Ultra-compressed adaptations of anything are normally a bad idea, but this was a shoujo manga and not plot-driven to start with. It's about emotion, not action. Thus, surprisingly, this anime stands up pretty well. It works. It's a successful telling of this storyline, never giving you the impression that you're seeing the crunched-down version. There's only one place where this affects the storytelling, I think. Yori takes a certain big decision based on his interpretation of Iku's feelings, but we haven't seen enough in the anime to justify his conclusion and so you might well assume instead that he's inventing weird stuff in his head. That was my assumption, anyway. (I assume the manga sets that scene up properly.) That said, though, Yori's hardly stable or well-balanced at the best of times, so "he's a presumptuous nutter" is a coherent, satisfying reading in its own right and the scene works.
Instead, you can feel the rhythm of the original manga by tracking its emotional beats. Quite a lot happens. There are quite a few story peaks where you're half-expecting the credits to roll... but then the story gets developed further and the characters find somewhere new to go. That's quite nifty. It makes the story and its cast (especially Iku) more dynamic than you might expect.
What really makes the OVA soar, though, is the laughs. It's not going as far in that as Oreimo, which knew it was a comedy, but it's still vivid enough that it wouldn't be surprising to learn that it was a direct inspiration. Both Yori and Iku can make your eyes pop out of your head. It's not just what they do, but what they say. They have friends at school. Yori even acquires a girlfriend, whom he'd rejected repeatedly until eventually saying "yes" in an attempt to get Iku out of his head. In other words, Yori and Iku don't exist in a vacuum and there are people in front of whom it would be wise to behave normally. A lovers' tiff? No no no no. Not a good idea. I was in stitches. These siblings have the self-preservation instincts of a lemming. There's a laugh-out-loud moment just from Dad coming home downstairs. Yori's out-of-the-blue confession to Iku had my eyes popping out (and Iku's in similar danger, seeing how huge her eyeballs become just then).
Yori's less spacey than Iku. "Not here; Mum and Dad will get suspicious." However that's not saying much, since Iku's behaviour is at times on a par with taking out adverts in the newspaper.
It's brilliant. Oh, and there's at least one point where I'm sure the comedy is deliberate. Look at their father's delivery of "I don't want Yori to leave" near the end, as if he's one of the girls in a harem anime. The anime's only problem is that everyone's manga-style faces look similar, so I ended up having to distinguish the girls by hair colour. Iku's a redhead, while Tomoka's toffee brown.
In the manga, apparently it's revealed that Yori and Iku are actually a case of heteropaternal superfecundation. Which must make everything okay, ahem.
It's great. Obviously you couldn't show it to most people, given the subject matter. However I'm pretty sure it would kill at, say, an undergraduate party. People would be laughing too hard to breathe. Besides, there's endless fun to be had with incest stories. The subject matter's so explosive that you can pretty much blow the audience's minds at will, while there's always the threat of the ultimate reset button. If either Yori or Iku starts thinking this is a mistake and they shouldn't be doing this... well, they're right!
I don't believe it's been released in the West, amazingly enough. I'm looking forward to seeing the live-action film, but I'm not expecting it to be half as funny.