Yuu-nee to Issho

Main character is a student in a provincial town Fukazawa. His father goes out for some time and mother drops down from accumulated fatigue, so protagonist suddenly finds himself in charge of small hot springs inn which is family business. His childhood friend luckily agrees to do the laundry, but the real surprise comes when he encounters an unfamiliar girl who claims to be his older sister Yuuno.

[From vndbreview]