He was probably Tardu's son and the governor of the northern provinces of the empire during the reigns of his nephews.
(see Göktürk family tree) The Western Turkic Khaganate in present-day Turkestan was founded as the result of the partition of the main empire after the death of Tardu in 603.
In 630, the Dulo clan of tribes together with the Karluks revolted against Tong Yabgu Khagan, the most successful ruler of the empire.
Külüg Sibir murdered Tong (who was his nephew) and declared himself as the khagan with the support of the Dulo clan.
[1] However the Nushibi faction didn't accept his title and supported Tong's son to throne.