In the German Democratic Republic, he served as Trade and Supply Minister, ambassador to Poland and, most notably, as the longtime head of the International Relations Department of the Central Committee of the SED.

[1] Following distance learning at the SED's Central Institute for Socialist Economic Management, Sieber served as Minister for Trade and Supply from March 1965 to November 1972, succeeding Gerhard Lucht, and as Ambassador of the GDR to the Polish People's Republic from 1973 to 1980.

[1] In December 1980, he was made head of the Department for International Relations of the Central Committee,[1][3][4] succeeding Egon Winkelmann,[4] who became Ambassador to the Soviet Union.

[5] The International Relations Department was responsible for preparing Politburo decisions that concerned foreign policy issues and to control their implementation.

At its last session on 3 December 1989, the Central Committee elected Sieber to a commission tasked with analyzing the causes of the crisis in the SED and in society.