What is the relationship between Cassius and Brutus in Julius Caesar?

Cassius and Brutus are brothers-in-law. But they are also senators and respected leaders in Rome, and this is the truly important aspect of their relationship. In Act I, Scene 2, Cassius first broaches the idea that Caesar is threatening to usurp the throne of Rome, ending the Republic. He appeals to Brutus's sense of patriotism as well as his personal pride by describing Caesar as a dangerous, designing man. All of this is intended to, in Cassius's words, be a "seduction" of Brutus. The "seduction," as it were, was a success. Brutus not only joined the conspiracy against Caesar, but he quickly became its leader, and, after his speech to the Roman people following the assassination, its public face.  It is their relationship as leaders of the conspiracy that is important to the development of the plot. They have a bitter argument in Act IV, Scene 3, but are reconciled, and both men die by suicide (with the help of their servants) after their armies are defeated at Philippi.