So our foe in Dol Guldur truly is Sauron. Olórin was the one to find this out, in a rash move so befitting his family. Why Manwë insisted on sending the older brother of that miserable brat I will never understand. I was the only one who volunteered, and I will be the one who sees my enemy laid low. Am I not the one who studies the furthest into his devices?
I must admit victory will be sweet. Long have I wished to humble that arrogant young Maia who always watched me with contempt in his eyes: as if he had anything to be contemptuous about, the traitor. If only there was some way I could humble him, control him, bring him back for all of Valinor to laugh and scoff at, with all finally knowing that I am the greater of the two…
Perhaps if I had the One…No, that would be far too dangerous. But then again…Sauron would have to obey the one who held his Ring, for he poured far too much of himself into it. If I had it, I could have what I wish…If only I had the One…The One…
So, as always with me, Gandalf and Sauron are brothers. Also helpful to know is that Saruman is older than both of them, and Curumo and Mairon never got along back when they were both serving Aulë.
This idea comes from the fact that Saruman starts searching for the Ring in T.A. 2851, approximately 150 years before he starts using the palantír and becomes ensnared by Sauron.