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 confesses 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?