Since then she has occasionally been seen accompanying her brother on his "field guidance trips". Sunday's outing was different as she was, for the first time, officially listed by her name and as a "senior official" attending the voting function along with several leading party and army luminaries.

State TV footage showed Kim in a black skirt suit, walking closely behind her brother and casting her vote into a ballot box.

Her precise position was not detailed, but she is believed to be the events director in Kim Jong-un's Secretariat Office.

In 2012, Kim Yo-jong was seen on state TV riding a white horse - a common propaganda symbol associated with the Kim family - with her aunt, Kim Kyong-hui.

As Kim Jong-il's sister, Kim Kyong-hui was an enormously powerful and influential figure who was given the rank of a four-star general. Together with her husband, Jang Song-thaek, she was seen as the power behind Kim Jong-un's throne, until the young leader had Jang purged and executed last year.

Ahn Chan-il, head of the Seoul-based World Institute for North Korea Studies, said Kim Yo-jong was being groomed to play the same supporting role as her aunt.

"Kim Jong-un and Kim Yo-jong will work in a similar way as their father and Kim Kyong-hui did in securing the future of the Kim dynasty," Ahn said. "And Kim Kyong-hui will eventually leave official life as part of the power shift in the family."

Kim Kyong-hui, 67, has barely been seen in recent years, with reports that she was seriously ill.

The Kim family has ruled the country for more than six decades with an iron first wrapped in a pervasive personality cult.