Brothers Conflict: White Wave

Ema had a younger step-sister when she was growing up named Mamoru Kazumi, but after a misunderstanding that nearly killed her, Umi left with her mother after their parents divorce. Years later, Umi is forced to leave the only place she ever considered a home when Rintarou discovers her situation. What will life be for Ema and Umi now that they have to live together once again?