Nancy Tidd, 61, and Emma Tidd, 40, each were shot in the head with a shotgun Monday, the Washington County sheriff's office said.
Washington County Prosecutor Michael Spahr said he would charge Christopher Sturm with juvenile counts of delinquency by murder and delinquency by aggravated murder. A 12-year-old cannot be tried as an adult in Ohio.
Deputies said Sturm, who was in juvenile detention, told relatives he shot the women.
"From the information we have, the boy was upset at the grandmother, and that led to the shooting," said detective Jeff Seevers of the Washington County Sheriff's Department.
The Tidds were each shot in the head around 5 p.m. Monday. The bodies were transported to the Montgomery County coroner for autopsy.
Investigators said the boy divided his time living with his mother and grandmother. He was found at his mother's home about six miles away in Lower Salem, which is 90 miles southeast of Columbus.
The grandmother was unable to defend herself because of a medical condition, and there were no signs of struggle, Seevers said.
According to local TV reports, Seevers of also said, "What our crime scene is showing us is inconsistent as to what the boy is telling us."
This isn't the young suspect's first brush with the law, according to WIS-TV, a Columbia, S.C., station. Sturm reportedly has pending charges against him for sexual abuse.
The bodies were discovered by a relative who went to the house to check on the two women.