Originally posted by Turnbull:
Tom told Michael that Frankie "thought you'd betrayed him." That speaks more to "get even with Michael" than "save my ass."
His brother could have been coerced. But I think the main purpose of his brother at the hearing was to shame Frankie into remembering the old ways of not cooperating with the government.
I also think that when he sees Michael sitting and talking to Vincenzo, he realizes it was Roth that set him up, not Michael. It also sets up the scene with Hagen, and his suicide.
