Kim Jong-un's sister, Kim Yo-jong, is sitting in one of the most important political posts in the regime.
Speaking to lawmakers at the National Assembly on Tuesday, South Korea's Defense Minister Jeong Kyeong-doo said Kim Yo-jong has taken over the party's Organization and Guidance Department.
The OGD is responsible for checking around three million party members to ensure they are following the teaching of the North's regime and remain loyal to Kim Jong-un.
Experts also believe Kim Yo-jong has carved out a key role in North Korea.
"Kim Yo-jong, the first deputy director of the department, appears to be a partner in charge of not only the regime's external sectors, but also overall state affairs."
Seoul's defense minister said Kim Yo-jong's official title would be first vice director of the party's Organization and Guidance Department.
And, she has a key role in shaping the North's relationship with South Korea and the U.S.
He added Seoul would respond with military action if the North launched a military provocation against the South.
Jang Tae-hyun, Arirang News.