Aniyome ~Nee-san wa Ore no Mono~

Protagonist "Takumi" and his older brother lived together with their parents. For years his brother, being chosen first and good at everything, fed Takumi's jealousy complex. When his brother got married to beautiful "Ayumi",, Takumi couldn't take it any more. So at his first opportunity alone with her, he seized the day. He seized his sister-in-law.

[From DLsite English]