Gaius Cassius Longinus ( before 85 BC â `` October 42 BC ) was a Roman Senator , a leader of the plot to kill Julius Caesar , and the brother in-law of Brutus . Gaius Cassius Longinus ( before 85 BC â `` October 42 BC ) was a Roman senator , a leading instigator of the plot to kill Julius Caesar , and the brother in-law of Marcus Junius Brutus .