Don't call me "Oniichan".

One day, his free-spirited mother, who had left him behind so long ago, suddenly reappears and asks him to take in a sister who is 12 years younger than him…! Now the older brother (24 years old) and younger sister (12 years old) are suddenly forced to live together, going from strangers to family…? (Source: Houbunsha, translated)