I always thought that there were implied threats to Frank about his brother and that his brother was used to shame him.
So if Frank testified he would be greatly shamed in front of his brother and his brother would lose face back home in Sicily. There also could have been the unstated threat from Michael that "if we can't get to you we can always get to your brother".
Of course Tom and Michael wouldn't have openly threatened Frank's brother, otherwise why would he have shown up? They probably told him that your brother's in a bit of a bind and he needs you for support...
"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming
Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.
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