According to Foreign Policy (Syria Is Now Saudi Arabia's Problem) Saudi Arabia has - under pressure from the US - taken over from Qatar as the main force controlling Syria's rebels. They are believed to have pressured the last SNC conference to diminish the influence of the Muslim Brotherhood and to increase the influence of Idriss.
Postscript July 2013: According to Al Monitor (West, Saudi Arabia Step Up Efforts to Arm Syrian Rebels) After three days in Istanbul last week, the National Coalition seems more like a coalition of Syrian tribes and north-east families. The tribe of al-Shimri, which is Jarba’s tribe, belongs to the area of Hasakah. Jarba’s deputy is Salem al-Maslat, a sheikh from the al-Jubur tribe in the al-Jazeera region of Syria. His other deputy is Mohammad Farouq Tayfour. He comes from a prominent Hama family and is a main Brotherhood figure. Suhair al-Atassi, despite her utter failure in her previous position and the harsh criticisms against her, was made a third vice president to the National Coalition’s president to strengthen the tribal and familial alliance and to show solidarity with besieged Homs. The new opposition leadership is a conglomeration of Syrian tribes and Saudi Arabia enjoys significant influence among them.