oniichan to issho by tokeino hari
sakura was 14 when the grandmother who raised her died, living her all alone. and then her 4 brothers [who had a different mother] appeared, and somehow she starts living with them!! having no blood relation with them*, sakura is bewildered by the sudden existence of her "oniichan".
* the 4 brothers' father is sakura's stepdad, i believe... i'm not 100% sure since i haven't read the beginning.