Cordelia McCarthy (left), 13, was last seen at around 9.30am on Tuesday 20 February at school in the Conisbrough area of Doncaster.
She was last seen wearing her school uniform but could possibly have changed her clothes.
She is described being around 5ft tall, with long ombre hair and is carrying a large Nike backpack.
Her sister Erin McCarthy(right), 16, was last seen at around 8.45am on Monday 19 February in the Bentley area of Doncaster.
Erin is around 5ft 4ins tall, of slim build, with red/brown hair that she normally wears in a bun. She was last seen wearing a blue duffel coat.
Officers believe the sisters could be together but are urging anyone who has seen either Cordelia or Erin to get in touch.
Please call 101 quoting incident number 1145 of 19 February 2018.