SEOUL, South Korea — In the highest-level contact between the two Koreas in years, President Moon Jae-in of South Korea plans to meet Saturday with the sister of the North Korean leader, Kim Jong-un, and the North’s nominal head of state, officials said on Thursday.

Mr. Moon’s luncheon meeting with the North Koreans was announced on Thursday, as Vice President Mike Pence arrived in South Korea carrying a message from Washington: not to waver from strong sanctions against North Korea.

The high-level North Korean delegation, headed by Kim Yong-nam, the North’s ceremonial head of state, will arrive in South Korea on Friday on a special plane to attend the opening ceremony of the Winter Olympics later in the day. The trip’s significance was underscored by the revelation on Wednesday that the delegation would include Kim Yo-jong, the only sister and a trusted aide of Kim Jong-un, the supreme leader.

In North Korea’s dynastic system, which upholds the Kim family’s “bloodline” as a sacred leadership qualification, Ms. Kim wields a singular clout that cannot be matched by any nonrelative members of the elite, regardless of their official titles, according to North Korea analysts.