Toilet Girl: Kichiku no Ugomeku

Yuuya's almost in heaven. His dad recently got remarried to the woman next door, and now his two childhood friends are living under one roof with him as sisters. But things quickly turn hellish when a new student transfers in. He's got enough power and influence to turn any female he meets into his personal toy, including the women in Yuuya's life. Sometimes, bigger really is better.

(Source: MU)