Bokura no Koi wa Shi ni Itaru Yamai no You de(2010)

Bokura no Koi wa Shi ni Itaru Yamai no You de

As children, twins Umi and Sora became acquainted with a four-year old Beniko. Years later, Umi has moved away, and Sora confessed to Beniko that he'll propose to her when she turns 18. But when Sora dies in a traffic accident, why does Umi appear before Beniko looking and acting just like Sora?