Responses
It wasn't yet revealed who the brother is, all we know is that Yuki's mother told him to find Omi-san. Could be because Omi is Yuki's brother himself or (more likely) because the mother knew Rou, Yuki's brother, was working under Omi-san and would only listen to him, so Yuki had to gain Omi's trust first.
Wait... isn't Yuki's brother Omi and not Rou? or did I mixed it up?????? Someon please clarify this for me (〜￣△￣)〜