Onee-san no Jijou (Sister's Situation) is about a 16-year old high schooler named Tsukiko. She has a crush on a guy in her class, who is also a popular rookie soccer player named Kou. The thing is, her mom just got remarried... to Kou's father, making Tsukiko and Kou step-siblings.
Yuu Asagiri
MangaSlick