How are Brutus and Cassius related, or linked together? And why does Cassius want Brutus's help in the conspiracy?

Cassius is a high ranking general so he would know Brutus who is in Caesar's inner circle. Cassius is weary that Caesar is becoming too powerful in Rome. He manipulates Brutus in to believing that Caesar must die for the good of the republic, for the good of Rome. Brutus is one of Caesar's closest confidants so Cassius knows he will be needed to bring Caesar down.