やっぱり妹がすきっ！

The main character is an ordinary high school student. His mother passed away when he was a child, and he lives with his father, currently lives separately due to business, and younger sister, Rika. One day, a girl, Aki, comes to his house, and insists that she is his younger sister. He calls his father to ask about it, and his father says it's true. At first, Rika is happy but she gradually gets mad because Aki behaves like his girlfriend. Aki's approach to him still continues, and Rika starts to approach him as well....