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Harris County Precinct 4 Constable's Office said the shooting happened on Saturday at about 2:44 p.m. outside the home improvement store at Cypresswood and 249.
Investigators said a 16-year-old with special needs got a hold of their parent's gun, which was in an unlocked box under a seat in the car.
The teenager accidentally shot their 14-year-old sister in the backseat, according to deputies.
The 14-year-old was taken to the hospital in an unknown condition.
No charges have been filed, and an investigation is ongoing.
Editor's note: Deputies haven't disclosed the 16-year-old's gender.