In The Godfather 2 Frankie Pentangeli is before a committee ready give evidence against Michael Corleone. His brother enters the room and Frankie to immediately changes his story.
Was it just out of guilt or was there another less obvious reason?
There's another less obvious reason that was left out of the final cut of the film.
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. His brother's presence, as well as the stare they exchanged, serves as a threat that if Frankie follows through with his planned testimony, retribution will be taken against his children, who are living in Sicily under Vincenzo's guardianship.
I would assertively say that the capture of Pentangeli's brother by Michael may very well serve as a sign that Hyman Roth is on the dead list. That is to say, Hyman has lost all his power but Pentangeli, whose brother has been deprived of Hyman's protection (or surveillance) and exposed to the killing muscle of Michael. Your brother is on your enemy's hand and so are all your other odds. The only way out is to get what your enemy wants.