your responses to my post was so great, i thought i'd try another one (actually 2):

did frankie flip because he thought michael was trying to kill him or was it just to save his ass?

when his brother was brought to the hearings and frankie decided to clam up, was it because: he thought his brother was in danger at michaels hands OR he was reminded of the "old rules" by his presence and rethought his unethical decision to turn on michael.

Being an "old school" kinda guy, I don't think that simply saving his own ass would have been reason enough for Frankie to flip.

With respect to Frankie's brother, it does not appear as though he was coerced into coming, and he kind of casts Frankie a dirty look when he sees him, rather than a look of fear.

Michael also gives us a clue, if you can take what he says to Kay at face value, when he tells her "It was between the brothers, Kay."

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.

The fact that Michael had access to Frankie's brother was enough of a threat to get Frankie to clam up.

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.

This was chopped from the original screenplay:


http://www.dailyscript.com/scripts/godfather2.html

KAY
I didn't even know he had a brother.
And where is he now?

MICHAEL
On a plane back to Sicily.

KAY
And that's all he had to do. Just
show his face.

MICHAEL
That's all. You see, in Sicily, in
the old days... there was only one
legitimate reason to kill a blood
relative... only one. IF he was a
traitor.

KAY
You would have killed his brother?

MICHAEL
Kay, you've got it wrong. That
kind of thing's all over, I promised
you. This was between the two
brothers. Years ago Frankie had a
young girlfriend; he called her his
co-wife. That was his joke, but he
meant it. He wouldn't divorce his
wife... because she was a great
cook. He said he girlfriend made a
spaghetti sauce once and it was so
terrible he knew he could never
marry her. He set her up in a
house in Jersey. She had to be
faithful... and she had to have kids.
And she did, two, a boy and a girl.
He had her checked out and watched
so she couldn't cheat... but the
girl couldn't stand that kind of
life. She begged him to let her go.
He did. He gave her money and made
her give up the kids. Then Frankie
took them to Italy, and had them
brought up by his brother Vincenzo.
Where he knew they'd by safe.

Kay begins to realize.

MICHAEL
When he saw his brother in the
hearing room, he knew what was at
stake.
(pause)
I don't think Vincenzo would have
done it. He loves the kids, too.
Omerta, Kay. Honor, silence. It
had nothing to do with me. It was
between those brothers.