Sister Contrast!

The main character has no sister. His parents divorced when he was a boy, and he lived with his mother since then. And his mother passed away last year. One day, he receives a letter from his father, and goes to his house. On his way to the house, a flash of lightning strikes him. When he wakes up, there is a girl looking at him. It seems she is his younger sister. He doesn't know what is going on, and goes to his father's house. There isn't his father at his house, just three girls await him. He starts to find out that the world he is in now is another world, and he is supposed to live with the girls in another world. His new life with the girls starts now....