Tsuki Yadoru

Tsuki Yadoru summary:

Dramatic series about Keiji Mogami and his half-brother Naoya Amamiya who is suddenly adopted into Keiji's family which highly prizes martial arts prowess. Naoya is several years older, and therefore better at swordcraft than Keiji, but Keiji's mother pushes him to work hard in order to beat Naoya. Naoya himself seems nice at first, but then he begins to show Keiji a much, much darker side.