The incident occurred at 6 pm on May 30 when Hau A Tuc, 23 and his 12-year-old sister, Hau Thi Mo, were walking home.
"As they were crossing the shallow Kham Spring, a flash flood suddenly descended and swept them away," local authorities said. "Some people tried to save them but failed due to the strong water currents."
Mo's body was found on Friday morning, while local authorities are still searching for Tuc.
Some northern provinces have seen heavy rains since Thursday afternoon due to the effects of a tropical depression. Some localities recorded rainfall of over 100 mm from 7 pm on May 30 to 3 am on May 31 including Dien Bien (150 mm), Hoa Binh (125 mm).
Late Thursday afternoon, a flash flood also caused serious flooding in many areas of the neighbouring province of Yen Bai.
Heavy rains will continue throughout the northern and central regions on Friday.
