Sophia True, 6; and Juliana Brown, 9, died Tuesday in the accident in Cass County's Howard Township. Penelope True, 4, was in critical condition.
Their babysitter, Sarah Michael, 19, was driving the girls home from school, less than a mile away, when the collision happened.
Sophia and Juliana were students in the Niles elementary schools, while Penelope was to start preschool this week, said Richard Weigel, superintendent of the Niles school district.
Michael bypassed a stop sign and her car was struck broadside by a pickup truck, according to state police.
Michael was treated at a hospital and released. The pickup driver was treated at the scene.