... were children of the same mother, Livia Drusa, and different fathers.
Cato's father, Marcus Porcius Cato, married Livia Drusa and had two children by her, Cato and Porcia who became the wife of another of Caesar's enemies, Lucius Domitius Ahenobarbus.
Livia had been married to Quintus Servilius Caepio and had three children by him, Q. Servilius Caepio who eventually adopted Marcus Junius Brutus (he had only one daughter, by his wife Hortensia), Servilia the wife od L. Licinius Lucullus, and "the" Servilia.
Cato's family is an example of the interesting alliances in the period :-)