In October, she became the youngest member of the Politburo, the main decision-making body of the party. She sat in the front row of a leadership platform in a party meeting in December.

Mr. Kim has an older brother, Kim Jong-chol, who has never been seen in the North Korean media and whose role, if any, remains a mystery. He was last spotted at an Eric Clapton concert in London in 2015, but little else is known of him. Ms. Kim and her brothers studied in Switzerland as teenagers.

Kim Jong-un’s older half brother, Kim Jong-nam, was killed in an airport in Kuala Lumpur, Malaysia, in February last year by two women who smeared his face with a rare nerve agent. The assassins, who are on trial, were hired by North Korean agents who analysts said were sent by Kim Jong-un to eliminate a potential threat to his dynastic grip on power.

Outside officials and analysts consider Ms. Kim the pre-eminent female face of the Kim family, which has ruled North Korea since its founding seven decades ago. Her father, Kim Jong-il, also allowed his own sister, Kim Kyong-hui, to hold prominent jobs in the government until his death in 2011. But she disappeared from view after her husband, Jang Song-thaek, was executed in 2013 by Kim Jong-un on charges of plotting a coup.

“Although Kim Yong-nam is the official head of the delegation, it will be Kim Yo-jong who will decide its activities,” said Cheong Seong-chang, a senior analyst at the Sejong Institute in Seoul, South Korea. “By including his sister in the delegation, Kim Jong-un may want to show his interest in improving ties with South Korea.”

Word of Ms. Kim’s visit came as Vice President Pence warned North Korea that it would face a new round of tougher sanctions.

“The United States of America will soon unveil the toughest and most aggressive round of economic sanctions on North Korea ever — and we will continue to isolate North Korea until it abandons its nuclear and ballistic missile programs once and for all,” he said Wednesday while visiting Japan.

But a day earlier, Mr. Pence had stirred speculation about a possible meeting with North Korean officials, saying, “I have not requested a meeting, but we’ll see what happens.” On Thursday, however, North Korea ruled out such a meeting.