From Manga Abyss :
This is a story about single parents who decided to get married and bring their children together under one roof, but the parents have not yet registered. One day, the parents disappeared and left the children together in the house. What will they do?
From back cover:
Because of their parents remarriage, high school student Satou Yuka suddenly finds herself living together with her stepbrother, 23-year-old high school teacher Satou Taichi.
Actually, their parents have not registered their marriage and therefore they aren't siblings by law; but since both parents aren't around, they end up living together!
Taichi, who has a serious case of sister complex, wishes for Yuka to call him "onii-chan", but Yuka's reaction is very cold...?

Volume 1 also includes a short story, "Private Princess".