Hitomi -My Stepsister-

June was rainy. My father came home earlier than usual and said that he's getting remarried.
Then, the following Sunday. Another rainy afternoon. Two women came to our house. One was a gentle-looking woman, the other... a girl with sorrowful eyes. Father announced them as my new mom and sister.
From that day on, life became a haze of confusion. To put it bluntly, I was lost. The girl who shyly glanced up at me and called me "big brother". I’d never properly talked to a girl before, so I had no clue how to handle this innocent, smiling sister.
Eventually, I realized I was thinking about her — Hitomi — constantly. Her pale skin and lips like sakura stuck to the back of my eyelids, flashing behind them every time I closed my eyes. So, that’s why I... assaulted Hitomi, my sister. It happened on a humid, rainy summer afternoon.

Translated from the official site