Recently, there have been a lot of imouto-based shows, and often getting more unusual by the season. This show is no exception as it notches up the Oreimo illicit relationship to almost Yosuga no Sora levels. Yuuya is a normal kid with a divorced father. His life suddenly changes when his father marries a single mother, who happens to have a young daughter. Unfortunately, his new step-sister is something between an anti-social kuudere and tsundere; thus Yuuya just ends up being ignored all the time. No worries! Just get a ghost of a forgotten childhood friend who harbored a secret crush on Yuuya to possess Mitsuki and off they go to the forbidden lands!
This ridiculous premise, which would only have worked in the hentai genre a few years ago, combines with many eroge text-based scenes (complete with VA sound effects) to complete the overt, ecchi "experience." If you're into shows like B Gata H Kei and OniSuki, this will be right up your alley. Just don't expect much in the plot department.
Animation:
The animation quality is fairly good overall with a fairly saturated palette and oh what the hell you know this show is all about the ecchi. Expect loads of ecchi and a huge amount of censoring for the TV version. Many scenes aren't even rendered to leave it to the imaginations of the viewer. Expect upskirts; a ridiculous, fetish-level amount of urination; breast grabs; onsen scenes; masturbation; etc. The sexual references extend all the way to a S&M chastity belt that collects happy juice. On the overall appearance scale, expect this to be a bit comical in style, something between Yosuga no Sora and OniAi. The character styling is fair with distinguishable characters (simply by bust size).
Sound:
The show opens with a playful tune that reminds me of Bludgeoning Angel Dokuro-chan (perhaps it's just the binkan reference), it isn't a particularly good melody but is a good match for the tonality of the show. The ED is a bit immature and the animation sequence is frankly a bit unsettling. The BGM is fair but also a bit on the immature side with the xylophones.
The VA work is pretty good overall, though again the emphasis is probably a bit more on the sound effects side than your typical show. Top honors go surprisingly to the dark horse candidates Aimi and Yasuaki Takumi for their realistic sibling portrayal, especially in episode 7. It probably is the show's fault but there's too much of an emphasis on personality extremes with either overly passionate or boringly dull characters.
Story:
The show follows a slice of life approach similar to Minami Ke with a fair amount of home as well as school scenes. The only difference is that small humor is replaced with big time ecchi as Mitsuki and ghost Hiyori fight for control over Mitsuki's body. Poor Yuuya is forced to try to rationalize why his new sister seems to love him at times and hate him at other times, while always wearing strangely kinky undergarments. Confounding all this, Yuuya's tomboyish childhood friend now returns as a neighbor and (gasp) she's now a babe.
The show is episodic for the most part with some adventures to water parks and onsens, but don't expect much in terms of event-based plot progression. It's a pretty light show so don't expect any life lessons or deep symbolism here. The story ends on a facepalm-worthy conclusion. which just confirms the seriousness (or lack thereof) of the show. I must say though, the whole setting is a bit too neatly tied in with the ghost not remembering anything other than her lust for Yuuya, believability of the show setting is absolute zero.
Character:
Yuuya is your typical nice guy who happens to be stuck with his new sister at home while his parents are out traveling, conveniently. He's not the most interesting of protagonists but gets the job done. His sister, Mitsuki, is probably the only dynamic character in the show; she slowly grows more accustomed to her new brother and starts to harbor feelings for him. Are they her feelings or residual feelings from Hiyori's possessions? Regardless of where they come from, this progression is definitely one of the more natural ones, especially considering the ecchi genre. Doesn't mean I have to like her cold attitude though.
Some of the supporting characters add a nice breather from the overly-ecchi main plot line while other supporting characters just add an unnecessarily mysterious aspect to the show. As mentioned before, Yukina and Moa (both supporting characters) are probably the most normal of all the characters. All the other female characters are somewhat crazy in one way or another; this was definitely a bit imbalanced for my tastes.
Value:
Definitely not something I'd feel comfortable watching with most people. And unlike Yosuga no Sora where particular episodes are particularly NSFW, this show has so many of those moments that the entire series ought to carry that label. While the development of Mitsuki was fair, the ridiculous premise and lack of actual motivation to keep watching (assuming you can't live on ecchi alone) bogged this show down. It's a light watch for ecchi lovers, nothing more and nothing less.