In William Shakespeare's Julius Caesar, act 4, scene 2, Cassius calls Brutus "noble brother," but the two are actually brothers-in-law, for Cassius's wife is Brutus's sister.
Further, Brutus and Cassius are both respected Roman leaders and close friends. Cassius, in fact, comforts Brutus and reminds him that he is an honored member of society. Brutus and Cassius are also both deeply concerned that Julius Caesar will assume a kingship that could end the Roman Republic, and Cassius is clear that Caesar does not have the qualifications to rule in that capacity.
Cassius is already scheming at this point. He wants to eliminate Caesar, and he wants Brutus to help him. But Brutus is hesitant. He is less emotional and more rational than Cassius, and he needs proof that his fears might come to pass. Cassius decides that he needs to motivate his friend, but he does so in a highly dishonest way. Cassius has some letters forged, making the documents look like they come from the Roman people to express their own concerns about Caesar's potential power grab and rule. Cassius then makes sure Brutus gets these letters.
Brutus believes that the letters are real, and he agrees to join Cassius's conspiracy. The two men thus become coconspirators and then murderers of Julius Caesar. They also become exiles when they are driven out of Rome and army leaders as they raise their troops to fight the Romans. They argue now, and their friendship becomes strained. By the end of the play, both Brutus and Cassius are dead.