The first Indigenous northerner to become a lawyer (LLB, 1975), and the second Métis in Canadian history to lead a constitutional government (after John Norquay of Manitoba), Sibbeston has been a determined and sometimes tempestuous advocate for Indigenous rights since he entered Territorial politics in 1970. As chairman of the Western Constitutional Forum, the group negotiating for the division of the Northwest Territories, creating the new territory of Nunavut, he insisted that the eastern boundary be placed so as to guarantee parity between the Indigenous population and the non-Indigenous population, most of whom live in the western region.
In 1984 he was appointed to the territorial council. As government leader of the Northwest Territory (1986-87), he launched and lost a lawsuit against the federal government, claiming that the Meech Lake Accord (see Meech Lake Accord: Document) violated citizens' rights. In 1988, as minister of economic development and tourism, he continued to oppose the accord. Sibbeston left active politics in 1991 to devote his efforts to territorial reorganization.