The incident happened at around 6.20pm on Friday when a vehicle left an unclassified road leading to Drumwhindle just off the A948 in the Mains of Arnage area, Ellon.
The vehicle was being driven by a 43-year-old man with two girls, aged 12 and 14 sitting in the back seats.
It is understood that the man is the father of the girls, police said.
All three have been taken to Aberdeen Royal Infirmary, where the 14-year-old remains in a critical condition.
The road was closed while officers carried our inquiries into the incident.