From Tokyopop:
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?!

Ever since the gate to the Youma world was opened, the Youma have been coming through into our world and wrecking havoc. Luckily, there is a police force specially designed to take care of such a problem. Sora is a newcomer who's stepping in to take over where his brother left off with his Youma partner Arashi. Can the two get along and really become true Youma detectives?

Note: Originally a one-shot-turned-series, it changed names to Omaga Police Sora to Arashi mid-run.