HONG KONG — Kim Jong Un has promoted his younger sister to North Korea’s most important political body, a rare public gesture of confidence by a leader whose dealings with family members are often opaque and sometimes deadly.

The sister, Kim Yo Jong, heads the propaganda department of the ruling Workers’ Party of Korea and is the only one of Kim’s siblings active in his government. She was made an alternate member of the Politburo on Saturday. Her promotion was announced Sunday by the state-controlled Korean Central News Agency.

Kim Yo Jong is believed to be 28 to 30 years old, which would make her the youngest member of the powerful but secretive decision-making body. Her promotion took place during a gathering of senior party members a day before the 20th anniversary of when the Kims’ father, Kim Jong Il, was chosen as general secretary of the Workers’ Party.

Kim Yo Jong is often seen accompanying her brother at state functions and on tours of military units and factories. Unlike senior generals and party secretaries who cluster around Kim Jong Un taking notes, Kim Yo Jong often appears in state news photographs able to move about on her own.

The siblings’ mother, Ko Yong Hui, was Kim Jong Il’s third wife. Kim Yo Jong’s bloodline gives her an unmatched status among the party’s elites. The state news media often shows top-ranking officials listening reverentially when she speaks.

Her promotion comes at a time of increased tension between North Korea and the United States, as Kim Jong Un tests nuclear weapons and ballistic missiles in contravention of international norms.

Her brother’s faith in her contrasts with his dealings with other members of his family, some of whom he is believed to have had killed.

Kim Jong Il’s first son — Kim Jong Un’s half brother — Kim Jong Nam, was assassinated in February at Kuala Lumpur International Airport, using a rare nerve agent banned by international treaty.

Kim Jong Il’s second-oldest son, Kim Jong Chol, was last spotted at an Eric Clapton concert in London in 2015, but little else is known about him. It is unclear why he was passed over for succession.

Despite the cult that has grown around the Kim family, little is known about its members. The state media sometimes shows Kim’s wife, Ri Sol Ju. And virtually nothing is known about their children; South Korean officials believe that they have three, the oldest of whom is believed to be 6.

Given his children’s youth, Kim may have promoted his sister to guarantee dynastic continuity should he unexpectedly be unable to govern. Kim Yo Jong’s ascension, however, is no guarantee of protection from her brother’s mercurial purges.

Kim Jong Un’s aunt, Kim Kyong Hui, a high-ranking party official and a close confidante of Kim Jong Il, has not been seen in several years and is rumored to be in ill health or under house arrest. Her husband, Jang Song Thaek, a four-star general, was charged with counterrevolutionary activities and executed in 2013.