The new power couple emerging from the shadows in North Korea

They're the power couple behind the throne of Kim Jong-il, North Korea's increasingly frail leader, a husband-and-wife team who may have edged closer this week to the centre of power in the reclusive Stalinist dictatorship.

While most of the attention this week is on Kim Jong-un, the Dear Leader's youngest son and heir apparent, his aunt and uncle are probably being groomed for a role as his mentors.

State media confirmed last night that Jong-un was elevated to the Workers' Party of Korea's Central Committee, following in the footsteps of his father, who was elected to the committee in 1972 – the first major step on his road to power. It was Jong-un's first known political post and followed his promotion to four-star general in the nation's all-important military.

Kim Kyong-hui, 64, who was also given four stars this week , is the ailing autocrat's only sister and daughter of the country's founding father, Kim Il- sung. Her husband, Jang Song-taek, 64, is now widely considered North Korea's number two. They were also both given significant political posts.

The couple are believed to have survived at least one purge and the death of their daughter, who reportedly took an overdose of sleeping pills in France after her parents demanded she end a relationship there and return home.

The appointment of Kim's immediate family to senior positions at the ruling party's biggest conference in decades is fuelling rumours that he is trying to extend the country's messianic leadership cult for another generation.

Kim Jong-il, 69, was also given a military role at the last Workers' Party conference in 1980, though he had to wait for his father, Kim Il-sung, to die in 1994 before assuming his current role. In the 16 years since, he has solidified his grip on the country by cultivating a quasi-religious cult around him and his father – the "eternal president".

The official announcements confirm pre-conference speculation that Jong-il, who suffered a stroke in 2008, would cement his family's hold on power before he dies. In June, Jang was appointed chairman of the National Defence Commission – the North's supreme governing body – making him the country's effective second-in-command behind the Dear Leader.

Kim himself was yesterday reappointed general secretary of the party with what state media called "the unanimous will and wishes" of the North's citizens. State-run television said that Kim, who attended the conference, had been "enshrined" by conference delegates, who "enthusiastically gave celebration with a storm of acclaim and the highest respect".

The Russian-educated Jang is thought to have been purged by his brother-in-law from 2004-06 – punishment for flaunting his opulent cadre's lifestyle. But he has since been allowed back into the Kim family's inner circle as the leader's health ebbs and he leans more on family members he can trust.

Jang and his wife met in the 1960s at Kim Il-sung University. He was expelled when the relationship became public but the pair continued to date despite the fierce opposition of Kyong-hui's father, and married in 1972. Both have been at the centre of power in North Korea for four decades.

Kyong-hui is thought to be, in effect, an economic enforcer for her brother in her role as director of Economic Policy Inspections. Defectors say one of her tasks is factory inspections and ordering the imprisonment and execution of failing managers and officials.

The presence of two powerful and ambitious mentors alongside the young heir Jong-un has raised concerns about a tussle for power. But the normally well-informed online newspaper Daily NK said this week that Kim's autocratic leadership made that "impossible", at least until after he is dead. "Rumours of a full-blooded power struggle are remote and show a lack of understanding about North Korea's reality," said the newspaper.

Pyongyang's neighbours in Seoul, Tokyo and Beijing are watching this week's conference carefully for signs of who will eventually run the world's newest nuclear power. Although few ordinary citizens have ever seen the young heir, he has been increasingly seen accompanying his father during his famous "on-the-spot" guidance tours to workplaces and the army.

But there appear to be few immediate signs of any softening in the hardline Stalinist stance. The party's official newspaper this week said that it would continue to stress the importance of Kim's "Songun" policy of putting the military first, and his father's philosophy of "Juche", or self-reliance.