Hajimete wa Onii-chan 2 ~Yua-chan to Sonogo~

This is an after story of the protagonist and Yua, one of friends of his little sister.
As opposed to the little sister, Yua is even-tempered and well-mannered girl.

When he has got to take a role of Yua's big brother for some reason, they start getting close.

[From DLsite English]