Born in 1988, according to the Unification ministry, Yo Jong is one of three children born to Kim's predecessor Kim Jong Il and his third known partner, former dancer Ko Yong Hui.
South Korea has been the target of a wave of angry statements recently from the North, which has accused Seoul of allowing defectors to launch leaflets critical of leader Kim Jong Un into its territory.
The leaflets -- usually attached to hot air balloons or floated in bottles -- criticise North Korean leader Kim Jong Un over human rights abuses and his nuclear ambitions.
North Korea did not answer a routine daily liaison phone call from South Korean officials for the first time in since 2018, South Korea's Unification Ministry said.