Pentangeli's Brother

In part, yes. His brother had already returned to Sicily, but bringing him to the hearing demonstrated that they knew where he lived and could get to him at any time.

You are right that he also received the assurance he needed from Tom Hagen that his family would be provided for after his death.

RIP Frankie Five Angels

Pentangeli's brother showing up had nothing to do with threats to anyone's life. It was all about honor. His brother showing up was a reminder to Frankie about the oath he was breaking and shame he was bringing to their family. How dare you disgrace and dishonor our family name and our father. The shame of that was enough to break Frankie down and get him to back down.

Frankie ultimately killed himself because he wasn't going to testify, didn't want to spend the rest of his life in prison (which he would have if he didn't testify), and Tom made it clear that if he did so, his family would be fine.

He didn't want his brother to see him turn rat. That's all it was.

I think it was because his brother was sure to tell everyone that his brother was a rat which is the lowest form of scum you can be in the criminal world. It's the code of Omerta: Never rat out your friends and always keep your mouth shut.

Good grief people, the only reason that Frank’s brother was brought to the trial was to signify to Frank that Michael had nothing to do with the hit, a symbolic representation of Michael’s absolute loyalty to Frank.