The girl appeared in juvenile court Monday, where she was placed on electronic monitoring and ordered not to drive. Her name hasn't been released. Prosecutors were granted 48 hours to file charges.
The girl was driving Friday with nine passengers, ages 12 to 15. The driver lost control and the vehicle rolled several times.
The Marathon County sheriff's department says the two 12-year-olds who died were Reighlee Stevenson of Oshkosh and Deserae Landowski of Wausau.
A third person remains hospitalized in critical condition. Seven others were treated and released from hospitals.
The Wausau School District and Horace Mann Middle School are making counseling services available.