All of a sudden your twin little sisters return!
The setting is a cafe that the protagonist's friend is having him run. That's where his twin step sisters come to live with him until graduation.
The older sister is the subdued Chiyori, and the younger sister is the cheerful and energetic Chika.
Both of them have feelings for the protagonist. Whenever he pays attention to one, the other gets jealous and sulks. It might seem like an envious situation from the outside, but it's a tough way to live!

[Translated from the official website.]