She was director of Relatives of the Detained-Disappeared organization, and campaigned to find out the truth about the people who were violently disappeared during Augusto Pinochet's dictatorship.

[2] When Augusto Pinochet overthrew the Chilean government in 1973 and assumed power, Sierra and her husband, Waldo Pizarro, a fellow communist, stayed in the country.

[1] After her husband and a friend disappeared after they were arrested by the police on 15 December 1976,[2] she became the president of a campaign group called Relatives of the Detained-Disappeared.

Even after a new government came to power, Sierra and other people with missing relatives were not told the truth and due to an amnesty agreement, those responsible were protected from punishment and so continued to campaign.

[1] After Pinochet was arrested in October 1998[1] Sierra traveled to London to help a Spanish prosecutor campaign for his extradition.