I see the possibility. Let me argue for another possibility. It could be a nominal sentence, elaborating the newly discovered fact. The reason I opt for this possibility is that, if this was a listing of those who are the object of "God has bestowed favor upon us" then the first part would have been "I, Joseph" rather than "I am Joseph."
So, you're saying the meaning is,
"I, Joseph, and this, my brother - God has bestowed favor upon us..."
In this case, there are two apposition nouns: "Joseph" and "my brother."
It is possible, but I suspect that the first part means "I am Joseph" because it's a reply to their question: "Are you indeed, are you Joseph?"