Shimai Souzoku

The main character used to live with his parents and three older sisters happily. But their parents passed away suddenly in a car accident and each child was accepted into different, new family. More than ten years later, here comes a lawyer and tells the protagonist that he has a grandfather and about his sisters' whereabouts. He goes out to meet his sisters soon but is disappointed to see them to have changed dramatically.

He and his three sisters gather up at their grandfather's house to allot his inheritance. The will says that the people who can receive the fortune by inheritance are only one sister and one brother and the certain sister is to be decided by the brother! One month is given to him to decide after living with his sisters. Who will be the one to receive the inheritance and what will happen to them at the end?