The finished film leaves unclear exactly what about his brother’s presence motivated Frank to change his story. The final film only states that Vincenzo is himself a Mafia chieftain in Sicily.
An early draft of the film explains that Vincenzo, shocked that Frankie is about to break his blood oath and betray the Corleones to government authorities, attends the hearing to remind Frankie that he must not break the Mafia’s code of silence, Omertà.