Actually FDR had a long history of admiration for Stalin and efforts to prop him up, including using taxpayer money to produce films presenting Stalin as being just wonderful.
But as to your specific question, first, there is no context making it okay to call someone who had killed millions under his dictatorship one's "brother." What would you think about "Hitler is my brother"? Really little difference.
But as you wished the context: FDR was bothered that he had traveled a great distance to accomodate Stalin and saw no signs that Stalin was pleased with him personally at all, so he came up with a plan that he would start making fun of Churchill, hoping to please Stalin in this way.
I now quote from the book Franklin Delano Roosevelt, by Conrad Black:
"A vague smile passed over Stalin's eyes, and I decided I was on the right track. As soon as I sat down at the conference table, I began to tease Churchill about his Britishness, about John Bull, about his cigars, about his habits. It began to register with Stalin. Winston get red and scowled, and the more he did so, the more Stalin smiled. Finally Stalin broke into a deep hearty guffaw, and for the first time in three days I saw light. I kept it up until Stalin was laughing with me, and it was then that I called him 'Uncle Joe.' He would have thought me fresh the day before, but that day he laughed and came over and shook my hand. From that time on our relations were personal, and Stalin himself engaged in occasional witticism. The ice was broken and we talked like men and brothers."
So it turns out that the quote is a paraphrase which more accurately should have been that FDR said he and Stalin talked like brothers.
However, inasmuch as Stalin had by even very early in FDR's Presidency murdered 10 million Ukrainians, and by this time murdered far more of those under his dictatorship, and FDR had never had a critical word (that I have ever seen) for Stalin and did propagandize for him, I think it's fair to say that he wouldn't have described his meeting and characterized his talk with such a tyrant as being "like brothers" except for genuinely finding kinship.
I've never seen anything from FDR that would indicate the opposite. Can any think of an example?
And it can't be said, "Oh, but the war was so terrible that being chummy with Stalin was the lesser of two evils and a great President was forced to make the wise choice," because Stalin had been murdering millions since well before the start of the war, during FDR's Presidency and what is the sign that FDR ever saw even the slightest problem with that?
I realize that Stalin (for some peculiar, if it is peculiar) reason has never among most people in this country acquired the reputation, in terms of gut feeling, that his deeds deserve.
So substitute in one's imagination "Hitler" for each case of Stalin, and imagine that Hitler had been murdering millions since the very beginning of FDR's Presidency and FDR gave Hitler the same press, and had a meeting and spoke the same of his meeting, as he did with Stalin.
Would we not then fairly criticize FDR severely for that? Would it not say something extremely shocking, in fact?
No different with Stalin.
As to whether the quote had to do with economic policy, I didn't say that it did. Though I can appreciate that since much of the remainder of the topic was economic, that could have seemed the intended point. The point was just a self-felt connection that FDR described regarding Stalin. Which I think the full quote supports: a self-felt real connection.
As opposed to FDR viewing Stalin as a murderous monster that he was forced to be diplomatic with. Nope, no sign of such feeling from FDR. Quite the opposite.
But yes, it turns out the quote was an imperfect paraphrase that has somehow acquired life as a quote, but is only close to the gist rather than being the precise words or really the exact gist. I apologize for my error there in mistakenly having thought of and presenting it as an exact quote.
Stalin was one of the most hideously murderous monsters and oppressors in history. If any have evidence that FDR objected to Stalin, his acts, or his form or manner of government on any moral, philosophical, or other level, let it be presented!
I would also wonder which plank of the Communist Manifesto FDR cannot be shown to have supported -- let alone one that he could be shown, from his deeds, to have been opposed to.
So that is why while many hope for Obama to prove to be another FDR, I hope that he will not.