Is the relationship between Brutus and Cassius genuine?

The relationship between Brutus and Cassius is primarily a political, rather than a personal one. Although they are friends and related by marriage their alliance is based upon their status as political actors within the Roman state. They come to share a common vision—a Rome without Caesar—but they differ in their motivations. Cassius resents Caesar for depriving him of the role in public life to which he believes himself entitled on account of his noble blood. Brutus, however, is more idealistic, seeing Caesar's dictatorship as a fundamental threat to the republican system he so much venerates and cherishes.
Whatever ties exist between the two men in terms of blood, class, and political ideology, there's no doubt that this is never really a relationship of equals. Initially, it's Cassius who's top dog. He cynically manipulates Brutus to get him to join the conspiracy against Caesar. He also plays on Brutus's famed nobility and his honorable intentions to persuade him to get involved.
After the assassination Brutus becomes the dominant partner in the relationship. Cassius stroked his vanity to get him on board for Caesar's murder, but Brutus's ego has given him an exalted sense of his own destiny. He disregards Cassius's sage advice at Philippi, with disastrous consequences. And though they are subsequently reconciled, their relationship only becomes one of equals when they both choose to die in the same way.