Iwane: Sword of Serenity

Iwane Sakazaki has two childhood friends: Kinpei Kobayashi and Shinnosuke Kawade. They received training at the same dojo. Kinpei Kobayashi's younger sister Mai is married to Shinnosuke Kawade. Also, Iwane Sakazaki will marry Mao who is the younger sister of Kinpei Kobayashi and Mai. The three friends are like brothers. Shinnosuke Kawade then hears a rumor that his wife Mai is having an affair and he kills her. Kinpei Kobayashi becomes enraged that his younger sister is murdered and kills Shinnosuke Kawade. To punish Kinpei Kobayashi, Iwane Sakazaki is ordered to kill his friend.
(Source: AsianWiki)