俺の妹は、ツンデレで変態でした ～お兄ちゃんの前でだけなんだからね！

The protagonist, Nagasawa Ikki, has a little sister named Nagasawa Mao. She's in a bit of a difficult phase of puberty. Mao is reliable, cheerful, openhearted, excels at her studies, and is a perfect superwoman! Ikki always gets treated strictly by his little sister, and even if he is her older brother, he's completely outclassed by Mao.

One day, as usual, Ikki is nearly late for school, and was struck by Mao to wake him up. While complaining, he went with his little sister and rode a crowded train together to avoid being late. The train was jam packed, which resulted to them sticking together and unable to move. At that time, whether its a coincidence or a prank from God, the train shook which caused Mao's breast to push into her brother. Even if she is his little sister, Mao is a beauty who is around his age, and as such, he started acting strange and was getting aroused. Mao noticed Ikki's change in behavior and said "Could it be that you got aroused?". Ikki was unable to answer, and with a devilish smile, Mao said "I'm your little sister, you know?". And then she reached her hand towards her brother's "swelled thing"......

[Translated from Official Site]