SEOUL, South Korea — The sister of North Korea’s leader, Kim Jong-un​, attacked South Korea’s presidential office on Tuesday, calling it idiotic a day after the South denounced the North’s first weapons test this year.

Kim Yo-jong, Mr. Kim’s only sister, also serves as one of his closest aides. She helped arrange the first summit meeting between Mr. Kim and President Moon Jae-in of South Korea in 2018, visiting Seoul with her brother’s letter of invitation.

On Tuesday, in the first​-ever​ statement​ issued under her name, Ms. Kim heaped scorn on South Korea, another sign that relations between the North and South have chilled since the collapse of a second summit meeting between Mr. Kim and President Trump​ in February last year​.

On Monday, North Korea conducted its first weapons tests this year, which involved large-caliber rockets. Mr. Moon’s presidential Blue House, called Chongwadae in Korean, ​immediately ​blamed North Korea for raising tensions. ​