Kim Yo-jong, the youngest daughter of late leader Kim Jong-il, will be replacing her aunt as a member of the Workers Party’s Politburo.
Kim Yo-jong (30), who has frequently appeared alongside her brother in public and is thought to have been responsible for his public image, was already influential as vice-director of the propaganda and agitation department. She was referred to as a senior party official three years ago.
Her promotion was announced by Mr Kim at a party meeting on Saturday as part of a reshuffle involving dozens of other top officials.
The BBC’s Danny Savage says the move to elevate Ms Kim will be seen as further evidence of the Kim family’s iron grip on North Korea.
When Ms Kim was given a key post at the country’s rare ruling party congress last year, it was widely expected that she would take up an important role in the country’s core leadership.