Troopers from the Franklin barracks announced the charges Tuesday against Zachary James Proper, of Oil City, but provided few details.
The Derrick in Oil City first reported the charges, which state police confirmed in a short news release.
A criminal complaint charges that Proper shot his grandmother, Dorothy Fross, and step-grandfather, George Fross, at their home in Sandycreek Township about 4 p.m. Sunday. That's about 70 miles north of Pittsburgh.
Calls to the boy's parents and a defense attorney listed in court papers were not immediately returned.
The boy has been arraigned and ordered held without bond, which is standard in homicide cases in Pennsylvania.