Pick of the Litter (2004)

Blurb:
Riku Fukagawa was found at the age of ten with no memory and no relatives. Despite this, he manages to make a good life for himself, until five years later, when he runs into his real family at a flea market. They whisk him away to their home, in another world called Yamato that strongly resembles the Japan of the Edo era. They explain to him that he, four of his five brothers, and two of his cousins run a store, the Hiyokoya Store, in the capital of Yamato. He doesn’t believe them, but because they’re so protective of him, he decides to take on a part time job at the store... Why are family reunions always so crazy?!

Source: www.anisearch.com/manga/3999