Appetite - Kyoudai Hiai ~Iya na no ni, Kanjichau~

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.
Now that his twin is too busy with a baseball tournament... armed with photographs he stealthily took when the two broke the taboo, driven by hatred towards his younger sister and jealousy for his brother, it's time to enforce equality.