Hajime-chan ga Ichiban!

Okano Hajime is an ordinary - or maybe below ordinary - girl who was forced to take care everything in the family since she was young because the Okanos were very poor. Hajime had five younger brothers, who were quintuplets, and they were scouted by a music company. Thus, the realistic yet dreamy (!) older sister had to try harder to watch over her restless brothers and to pursue her own idol, who was in the very same company.
Taken from M-U