Gibomai ~Haitoku no Kyouen~

A renowned painter died of heart disease while he was married to Megumi, a woman much younger than he was. His son, Kyosuke, never liked that union and wanted Megumi out of the house. Therefore Megumi proposes that Kyosuke can do anything he wants with her daughter Yuna, Kyosuke's sister-in-law, if he allows them to stay. Naturaly, Kyosuke accepts. [Edited from AniDB]