The shootings happened Saturday, the opening day of hunting season in Montana.
The siblings from Power were on a hunting trip with family and friends in the Missouri River Breaks about 70 miles south of Chinook.
Blaine County Undersheriff Pat Pyette says they had finished hunting for the evening Saturday and the girl was unloading her rifle when it discharged.
Pyette says the 17-year-old boy was shot in the face and died at the scene. No names were released.
Also Saturday, a 16-year-old boy was shot and killed in an apparent hunting accident near his hometown of Malta.
The preliminary cause of death was listed as an accidental shooting and an investigation is under way.