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

Ms. Kim 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.

Ms. Kim 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 Mr. Kim taking notes, Ms. Kim often appears in state news photographs able to move about on her own.