On Saturday, the North Korean leader made her a member of the regime's Political Bureau, state media reported.
Kim Yo Jong and Kim Jong Un have the same mother, Ko Yang Hui, a former dancer born in Japan who was believed to be the third partner of Kim Jong Il, who died in 2011.
The North Korean leader's sister, 28, is believed to be married to a scientist from a non-elite family.
Her first official mention in Korean state media came in 2014 when she accompanied her brother during elections for the Supreme People's Assembly. Kim Jong Un has since then been accompanied by Kim Yo Jong on "field guidance trips" and other political events.
She worked in a number of roles within the government, including being appointed to vice director of the party's propaganda and agitation department in late 2014.
She is also believed to be the inspiration behind Kim Jong Un's various visits to theme parks, schools, and friendship with the former NBA star Dennis Rodman.
Although she's held many roles in government, her latest promotion makes her a member of the Political Bureau, the country's top decision-making body.
The position has solidified her role as one of the two most visible women in Korean public life alongside Kim Jong Un's wife, Ri Sol Ju. The leader's sister may hold more power than the North Korean first lady.
Because of Ri Sol Ju's status as a commoner, she is not able to hold influence over important government decisions. Kim Yo Jong, although not be as powerful as her older brother, directs many major events and demands the respect of other officials.
In January, the U.S. Treasury Department blacklisted Kim Yo Jong for her role in human rights abuses.