Suppose Michael hadn’t located Pentangeli’s brother, or was otherwise unable to stop him from testifying. What would have happened to him?

A Congressional investigative subcommittee isn’t a prosecutor or court of law. But any testimony given under oath can be turned over to a prosecutor if the subcommittee thinks it contains something actionable. Had Frankie testified against Michael, Questadt would have recommended to the US Attorney for the District of Columbia (federal prosecutor) that Michael be indicted on five counts of perjury.

In order to bring in a perjury conviction, the prosecutor needs a primary witness (Pentangeli) and either a corroborating witness or corroborating evidence to back up the primary’s testimony. I’m guessing that his ace in the hole would be Willie Cicci. Wittingly or not, Cicci was the mainspring of the trap the subcommittee laid for Michael. Since Michael didn’t know that Pentangeli was waiting in the wings, Cicci’s testimony that he never got a direct order from Michael emboldened him to lie under oath. I’ll be the subcommittee deliberately didn’t press Cicci too hard for details—his purpose was to make Michael relax.

But the prosecutor would press Cicci hard for more details, and Cicci—whose life was in the hands of the Feds—would cooperate. He might have been pressed to recall the famous line to Tessio—“the boss says for you to go ahead”—as proof to link Michael to Tessio’s murder, as well as the others committed that day. He might have pushed Cicci to “think harder” about any contact he had with Michael on criminal matters. Law is a competitive business. When a case goes to trial, the figure of merit isn’t innocence or guilt, or even right or wrong—it’s about winning and losing. And, once a case goes to trial, it’s in the jury’s hands, not in law books. Guilt is in the eye of the juror.

Michael wasn’t without resources. He might have been acquitted; or, if convicted, won an appeal. But, look to the hidden hand of Hyman Roth. His objective in using Questadt and the subcommittee wasn’t necessarily to put Michael in prison (although they would have liked that); it was to destroy his “legitimate” reputation. Even if acquitted by the jury or on appeal, Michael would always be tarred and feathered in the public mind. The Nevada Gaming Commission would have put him in the “Black Book,” and his casino holdings would have been put under new ownership that had no connection to the Corleones. Guess who?

Cicci was probably the witness who could corroborate Frankie's
testimone. Without taking the fifth Cicci had already testified that he was a "button" and explained what it meant, and he jked that the family had a lot of "buffas," one of whom would have been Frankie. To get a conviction all they had to do was get Frankie to say Mike ordered people killed, and he passed the word to Cicci who would corroborate, no doubt under a plea agreement.

The perjury counts would be brought in the United States District Court in D.C., which is well known as favorble to defendants. Moreover, many juries do not trust witnesses who have flipped to get immunity, and Michael and his legal team could easily say the murderous Frankie and his friend Cicci
had turned on him. Michael had witnesses to Frankies demanding that he wanted the Rosato brothers dead, and Michael telling him to make the peace.

As far as the casinos, Michael owned very little stock in them, and I believe they were owned by straw men but controlled by the Corleones.

As for Roth, he'd be in a pickle. Obviously he would never testify that he ordered the hit on Frankie, and by the time of the Senate hearings Michael already knew that Fredo had betrayed him to Roth.

My guess is if Michael got acquitted, or more likely had a hung jury Frankie and Cicci would have mysterious accidents before there could be a new trial.

I think he would have kept the casinos, but his hoped for squeaky clean image would have suffered, but the bright side is that had his reputation been tarnished, there could have not been GF III.

I think Michael would have put all his political power to the test and after the cameras stopped the perjury inquiry would have been dropped. I don't think it would have gotten to the federal prosecutor or if it did he would have had some "legitimate" reason for not pursuing it.

But also regardless of what happened or how much it cost Michael would have dealt with Frankie.

Also by this point he knew the comittee's counsel belonged to Roth. This information would have been leaked out and could have headed off a trial. No self righteous congress person would want the word to get out that their own lawyer was controlled by Hyman Roth, and the whole thing may have died right there.

what exactly did tom say to frank pentangeli's brother after the court hearing?

He said that the family's honor was intact, "sta posto", which means "as it should be" or "everything's fine".

Did they threaten Frankie's brother? Or did they just tell him that he was about to break omerta? And was Frankie's brother's family part of the Sicilian mafia?

I always thought that the threat was implied and ambiguous.
Perhaps they told Frankie's brother that Frankie was under the gun and needed to see family support. The brother might not have necessarily believed that of course but it didn't really matter.

And of course Frankie seeing his brother there in Michael's presence would have assumed the worse.

if franks brother was an honorable man,as tom hagan implied at thew hearing,then it would go against the mafia code to kill him to spite frank,although with mike backed into a corner as he was i would not have put it past him

For years, I thought that the brother was brought there as a threat to Frankie - "If you testify against me, your brother dies." However, I've come to believe that the brother was there to remind Frankie of the oath they had taken. That's why Tom Hagen (in perfect Sicilian, btw, so kudos to Duvall) confirms that the family's honor is intact.

I don't think it had anything to do with reminding Frankie of anything except: you talk, your brother dies.

But with Michael in prison, would the Corleones (=Neri+Hagen) have killed Franke's brother? And if you're right, wouldn't he be a little more scared? At the hearing he looked a little angry.

I believe it was both: to remind Frankie of his "honor," and to imply death to his brother.