What exactly was Yosef telling his brothers that they did not already know?
Ramban: He was telling them that they had no reason to be upset at what they had done, because it was an act of Divine Providence, inasmuch as due to the two years of famine that had already passed, grain was so scarce and expensive that, if not for the fact that (thanks to their efforts), Yosef was in the position that he was, they would never survive the five years of famine that were still to come. 1
He did not know of course know, that the famine would cease with the advent of his father, but then that too, only occurred because he was there.
רש"י: להיות לכם למחיה: ממה בא רש"י להוציא?
גור אריה: אפשר לפרש שה' שלח אותי למחיה- שאחיה אני, ולכן פירש רש"י- 'לכם למחיה'.