Yume no Kodomo

Youji Takashima is a high schooler with a major 'sister complex' towards his older sister, Senko, 26, whom he's living with... One day, she introduces him to one of her old friend from college: Ren Akishima. Their first meeting doesn't go as expected, Youji doesn't like Ren and starts to think that he wants to marry his sister. Later, Senko reveals that she's leaving for NY to get some advancement in her job and that she intends to leave Youji with Ren.

(from EoP website)