ETA: At the time I (and the people below me) reviewed this series, it was incorrectly listed as completed at 2 volumes, causing us all to think it had an abrupt ending. It turns out there are a total of 4 volumes to the series. I'm leaving the review as-is, but will update it if and when I read the rest of the series. ^_^;
This was almost like a (platonic) love story between the brother and sister rather than the sister and the brother's friend, as one might expect. I was fine with that as the relationship between the two siblings was quite touching and not really meant to be incestuous. (Although Shirou idolized his sister and was extremely overprotective of her, so I suppose if you really wanted to see incestuous overtones there, you could.)
What was really interesting to me, but disappointing for many readers, I'm sure, was the fact that Shirou's friend, who Masago had a huge crush on, had absolutely NO interest in her, despite the fact that he knew about Shirou's spirit inhabiting her body and was constantly trying to protect her because of it. In cases like this, the male lead always falls for the female lead, and yet this guy didn't. This leads me to believe that the series was cancelled before anything could actually develop between them, thus the lame abrupt ending. It seemed to me that a possible love-triangle was being set up at the end with the friend, the delinquent boy, and Masago, but of course it never went anywhere because of the ending. Or perhaps the delinquent boy was supposed to be Masago's compensation, because at the end of the manga, Masago tells Shirou essentially that
the friend was actually in love with him the entire time. (The yaoi fangirl in me was fist pumping at that revelation. ^_^;)
Had the series gone on longer, perhaps the mangaka might have continued with that plot twist? Looking back on it, there were some indications of his feelings prior to the ending chapter, so instead of a love-triangle, we might have ended up with one heck of a love-quadrangle.
But unfortunately with the way the series is, all we can do is speculate. For what it is, the series isn't bad and is actually mildly entertaining thanks to some fun characters (thankfully Masago included). So read it if you run across it (no need to go out of your way to track it down), but don't expect brilliance or an especially satisfying ending.