One Plus One (FUJISAKI Mao)

From Lovers Dream:

It's about a girl who falls in love with her little brother. She thinks it's bad, but the two of them learn that they are not related by blood.

Mei and Takeru are twins who have a very close, if slightly flirty, relationship, and are quite popular within the school. Their relationship is both a source of amusement and dismay to both the male and female school population, especially with Mei, whose massive brother complex keeps her oblivious to serious confessions from other males. Their relationship becomes even more complicated, however, when the two of them learn that they are not related by blood.