Gibomai

犠母妹

A renowned painter dies of heart disease while being married with Megumi, a woman much younger than he is. His son, Kyosuke, never liked that union and wants Megumi out of the house. Therefore Megumi proposes that Kyosuke can do anything he wants with her daughter Yuna, Kyosuke's stepsister, if he allows them to stay. Naturally, Kyosuke accepts. (Source: AniDB)