However, there is a considerable difference between the manga and the movie. In fact, the two areal different that I am "forced" to review the movie in total isolation from the manga. Forced because I don't want to. I have always been the kind that pays respect to the original literature which is also the reason I often end up being disappointed with books turned into movies.
However, this movie is just so different, from the storyline to characterization that it has to be treated in isolation.
Yori and an Iku Yuki are twins but Yori falls in love with his sister, Iku. After holding back his feelings for so long it became unbearable that he had no choice but to cross the line from being siblings to lovers. However, as both realize that love may actually not be enough to keep their relationship going, they must make the decision whose hearts to break, theirs or the people around them.
Love always have a reason. That's probably the opposite of what we have all been taught about love but that's part of the tragedy of making movies, you always have to provide motivations for people to do things. Yori falling in love with his sister was one major event that needed motivation. Perhaps due to lack of time, the movie used one scene in the twins' childhood as the motivation for Yori falling in love.