Sophie, aged 17, was last seen leaving Altnagelvin Hospital in Derry just before 4pm on Thursday, August 12.
Sophie is described as being of slight build and approximately 5′ 3″ tall.
She and was last seen wearing light blue jeans, a grey and navy hooded jacket and black and white trainers.
Anastasia is described as being of slim build and approximately 5′ 1″ tall. She was last seen wearing a dark green jumper, black leggings and black shoes.
It is believed that both girls have travelled to the Belfast area.
Anyone with any information is asked to contact police on 101 quoting reference 1236 of 12/08/21.