At the same time his father-in-law managed to get him appointed one of the three Commissarissen-Generaal (Commissioners General sent to re-organise the Dutch possessions in the face of British and French incursions, as well as restive native rulers).

This duo managed to concentrate a lot of power in their hands, and those of their wider family attachments, especially after the new Batavian Republic merged the Commissie-generaal with the Raad van Indië (Council of the Indies).

In 1800, the Batavian Republic took over formal control of the government of Nederlands-Indië (as it was now called) and vested authority in Council for the Asian Possessions, (Raad der Aziatische Bezittingen) whose combined assembly was packed with family members.

In 1801, following the death of his predecessor, Pieter Gerardus van Overstraten, he became acting Governor-General, and, one year later, was confirmed in the full, official function by the Batavian Republic.

Siberg, however, remained in the Indies, as leader of the Oudgastenpartij, a group of conservative landowners and merchants, still influencing (or interfering with, or frustrating, according to taste) his successors' attempts at reforms.