I think that the role of Arsat's brother is both direct and complex. On the direct side, his role is to be the source of help and strength for Arsat. It is the brother that provides counsel to Arsat when he struggles for initially what to do and then it is the brother who offers him help when he realizes what he feels he must do. Arsat's brother is the reason that Arsat and Diamelen are able to run away together, as he offers up himself as the sacrifice to the pursuing guards.
In this characterization, there is also complexity. Arsat's brother represents the purest of the characters in the short story. His role is to thus become the compass by which Arsat and Diamelen are to be assessed. When Diamelen dies, Arsat calibrates his own sense of right and wrong to pursue vengeance for his brother. Arsat's brother's role is one of nobility, representing how the modern setting is one in which the purest of characters are not able to fully enjoy the benefits of life as they are the ones who end up sacrificing the most. In this, Arsat's brother occupies a thematic role that is of significant importance to this short story.
Initially, Arsat's brother is his emotional confidant. Arsat's brother advised him to be patient and wait until Si Dendring died before seeking a relationship with Diamelen. However, Arsat becomes impatient and confides in his brother, who becomes his co-conspirator as the two warriors plan on escaping the village with Diamelen. Arsat's brother selflessly risks his life helping his brother escape with Diamelen. When the war party begins closing in on them, Arsat's brother sacrifices himself by attempting to stall the search party. As Arsat and Diamelen escape unharmed, Arsat's brother is overtaken and killed by the war party. Essentially, Arsat's brother courageously risked his life for his brother's future. At the end of the short story, Diamelen dies, and Arsat seeks redemption by returning to his village to fight against the men who killed his brother. Arsat's brother plays the role of savior and is portrayed as a fearless, loyal individual who was willing to die for his brother's happiness.