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 North’s supreme leader.

In Kim Jong-un’s dynastic regime, which upholds his “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.

Ms. Kim would be the first member of North Korea’s ruling family to visit the South since the 1950-53 Korean War. Her father, the late North Korean dictator Kim Jong-il, held a summit meeting with the former South Korean president Kim Dae-jung, but did not keep his promise to visit the South a second time. The elder Kim died in 2011, bequeathing power to his third son, Kim Jong-un, who has rapidly accelerated the North’s weapons programs.

It remained unclear whether Ms. Kim would be carrying a message from her brother to Mr. Moon, who has said he is willing to meet Kim Jong-un if he is reasonably sure that such a meeting would help end the crisis over the North’s nuclear weapons and missile development.

The North Koreans’ trip has raised hopes within Mr. Moon’s government, which has sought to use the North’s participation in the Olympics to expand dialogue with Pyongyang and ease tensions over the North’s nuclear weapons program. North Korea is sending 22 athletes to the Olympics, as well as hundreds of cheerleaders, musicians, singers and dancers.

The two Koreas will march together in Friday night’s opening ceremony in the South Korean town of Pyeongchang, and even field their first-ever joint Olympic team, in women’s ice hockey. Mr. Pence also plans to attend the opening ceremony.

But hopes for détente have been tempered by deep skepticism over the North’s sudden overtures. United States officials fear that after a year of increasingly provocative missile and nuclear tests, Kim Jong-un may be on a charm offensive to undermine international sanctions that have begun biting his isolated country.

Before leaving Japan for South Korea, Mr. Pence said the United States would not be duped by any false overture.

“We’ll continue to seize every opportunity to ensure that North Korea does not use the powerful imagery and backdrop of the Olympics to paper over an appalling record of human rights and a pattern of developing weapons and conducting the kind of missile launches that are threatening our nation and threatening neighbors across the region,” Mr. Pence said.

Mr. Pence was meeting Mr. Moon on Thursday for dinner, where he was expected to urge caution in dealing with the North Koreans.

He said he would seek a South Korean commitment to “continue to isolate North Korea economically and diplomatically so that we can achieve what the world has longed to see, which is a nuclear-free Korean Peninsula.”

Mr. Moon has supported American-led sanctions against the North, but he has repeatedly advocated the need for dialogue. He has also been a vocal critic of any use of military force against North Korea, which is being considered as an option in the Trump administration.

With North Korea’s decision to send a high-level delegation, the Winter Olympics have suddenly become an arena for high-profile diplomacy. Although North Korea said its delegates had no willingness to meet Mr. Pence, the vice president again did not rule out the possibility of a meeting on the sidelines of the Games.

“There may be a possibility for any kind of an encounter with North Koreans, whether it be informal or whether it take the form of a meeting,” he said. “As I said, we’ll have to wait and see exactly how that unfolds.”