Appetite – Secret Sorrow of the Siblings English version VN
Kaito and Rikuto are twins and they have a younger sister named Arisa. Though they are twins, the nerdy Kaito is obviously inferior to the intelligent and athletic younger twin brother, Rikuto. One night, Kaito saw his younger sister enthusiastically fellating Rikuto. Watching the spectacle while feeling the sense of being left alone, he blazed up with rage.