Kanojo no Imouto ga Kyonyuu Mini Skirt JK de Koakumakei

The protagonist's life took an unexpected turn when he found himself on the same train as a high school girl who exuded an overwhelming presence. Her striking appearance, with a full figure and a short skirt, left a lasting impression on him. As it turned out, this girl was none other than the younger sister of his colleague, Sakurai-chan, who is a year older than him. Sakurai-chan, in contrast to her younger sister, is a serious and composed individual, leaving the protagonist at her mercy.