If it's a necessary clarification, use the commas. If not, don't.
I have three brothers: John, Paul, and George. I went fishing with my brother Paul.
(Since I needed to clarify that it was my brother Paul, I didn't use the comma.)
I have one brother: Paul. I went fishing with my brother, Paul.
(In other words, I went fishing with my brother, his name is Paul.)
And now that I've written "Paul" so many times, it's starting to look like a really weird name.