Is it as good as the anime? (I can't answer for Kotomi Aoki's original manga, not having read it.) Personally I'd answer that with a fairly comfortable "no", but it ends quite well and its story does eventually manage to find itself.
Its story is of course twincest. "Boku wa Imouto ni Koi o Suru" means "I'm in love with my little sister", which immediately raises certain questions.
1. "Good grief, are you joking?" (No.)
2. "No, seriously. Anime's one thing, but something like this surely wouldn't get turned into live-action. You're pulling my leg, right?" (No.)
3. "How exploitative is it?" (It's not. It's based on a girls' manga. There's not even any nudity, unless you count a bra.)
4. "Do they actually, um..." (Yes, they do.)
5. "What's the tone like? Does the film go all Humourless And Very Very Slow, or does it actually have some life? Does it manage to be funny?"
That's a more complicated one. Unfortunately Jun Matsumoto and Nana Eikura aren't good in the lead roles, but the film's later stretches play more to their strengths and in the end I thought it managed to do something worthwhile. It ends well.
It begins horribly. The director seems to have found the two creepiest small children in Japan to sit in a field. That must be deliberate. I was actually scared. After that, we jump forward to Yori (brother) and Iku (sister) played by Matsumoto and Eikura respectively, with Yori giving us yet another kind of creepy. That's a serial killer look. Of course that's because he's trying to resist his feelings and keep his sister at arm's length... but that's less clear in this version, because this first act is centred on Iku rather than Yori. We're seeing in her head, not his.
In fairness, this makes the story more plausible. We don't need an explanation for Yori's behaviour, but we most definitely needed one for Iku's "yes". This provides it. It also makes Yori less unlikeable, since Matsumoto's version of him isn't a massive dick with a hair-trigger temper. However it does turn Iku into a depressive anti-Iku. There's nothing carefree or even happy about her. She thinks she's stupid and incapable of anything.
This makes these early scenes dreary and joyless. Yori's being Yori, Iku's weighed down by her feelings right from the beginning and even the supporting characters might as well be being drowned in a ditch. Yano (Yuuta Hiraoka) is being rejected by Iku with no explanation, while Tomoka (Ayaka Komatsu) looks like a dead fish. (She's much prettier when she's smiling, but she's not going to be doing a lot of that here.)
We then lurch into the film's worst-realised scene, which is also unfortunately its most important. Yori's confession is still jarringly out-of-the-blue and not particularly plausible, but in addition the film mistakes "slowness" for "acting". Matsumoto and Eikura fail. The director plays it all out very, very slowly and the actors almost seem lost. They're not inhabiting the scene. They're not supporting their material. It gets boring, whereas of course it should have been terrifying and explosive. You start noticing the hand-held camerawork, because what's on-screen isn't holding your attention.
After that, the film starts improving.
Matsumoto and Eikura aren't creating tension with their acting, but the situation's doing it for them. They've slept together. Their choices have got dangerous. The film can thus get laughs from Mum at breakfast ("You're quiet; did you fight?") or from them walking down the corridor at school. They're not big laughs, admittedly, and nowhere near what I got from the anime, but at least it means the story's coming alive.
It also becomes clear that even if our leads can't really act, they are at least okay at being happy and natural. Matsumoto can do a nice line in "brooding", but I quite liked the scene of them just having fun on a bicycle together. For the first time, the actors seemed to have found some chemistry.
Eikura is also quite funny with her character's situation-blindness. She becomes a complete girlfriend, having unplugged all reservations, qualms or second thoughts. She'll do a girly whine when Yori's not being romantic enough or something. This made me laugh, especially when Yori's tying himself in knots and having the Tomoka sub-plot moved later in the story. Her e it comes after his confession to Iku, not before. This is dramatically stronger, but at the cost of making Yori look like a shit. He deserved to get dumped, frankly. (Okay, yes, we've entered a topsy-turvy world of anti-morality since the person who'd be doing the dumping is his twin sister, but he did.)
Yuuta Hiraoka is also pretty good as Yano, incidentally. He's actually acting, playing layers and motivations in his scenes with Matsumoto. (I think the character's secretly in love with Yori, personally, and the important question is what his feelings are towards the twin to whom he'd actually confessed. Did he love her too? He clearly has some feelings there, though, as is demonstrated in what would otherwise be the brain-throttling scene where he becomes the film's incest cheerleader.)
Ayaka Komatsu's also doing a decent job as Tomoka. (I liked her in Sabu's Miss Zombie too.) Mind you, she's suffering somewhat from the director's love of framing his characters in statue-like tableaux, standing distantly like soldiers on a parade ground, whenever he wants to express a lack of emotional connection between them. Tomoka's plot role means she gets that a lot.
Yuko Asano's underused as the twin's mother, though. She deserved to get more to do, especially since one of her first character actions is to discover something incredibly suspicious. (She's changing their bedclothes. Mind you, it could have been worse. She could have found blood or a wet patch.) This film isn't actually extolling Yori and Iku's choices, but it does end up taking no moral stance and simply observing how the story affects the characters. It's thus portraying a universe that's a good deal more incest-friendly than you might expect in real life. Tomoka doesn't freak out or denounce Yori as a pervert, for instance, but instead isn't discouraged at all in her feelings for him and instead sees an opportunity to try to get into his pants. Yori and Iku's mother would probably have had different things to say on the matter, but unfortunately she's been sidelined.
The ending's excellent, though. It's gentle, unconventional and ambiguous, allowing everyone in the audience to draw their own conclusions about what Yori and Iku have concluded. Have they agreed to face up to reality? You could debate that for quite a while. They try to return to childhood. There's a symbolic burial and talk of a metaphorical marriage. The piggy-back play is charming and silly, but it's leading into Matsumoto actually finding emotional depth and being moving. Both he and Eikura do well there. It's a quiet non-finale to a very slow film, but it fits and it gives you quite a few possible insights into the characters.
In the end, I liked this film. It's very slow, in that Japanese way that they've been doing for as long as there's been cinema. It has some acceptable acting, some of which even comes from Matsumoto and Eikura. I like how it characterises the two leads and the differences between them. "You lied." It even has some laughs (which I think is important). It's a long, long way from being flawless (both in acting and directing), but it's gentle and eventually manages to find emotional meaning.