The New Women Foundation in Egypt said two of the mother's brothers tied the girl to a chair at her home in Giza and began beating her while her younger sister watched, Bikyamasr.com reported Monday.
The girl, identified only as Iman, died on the chair and her body was bruised and bleeding, the rights group said without reporting when the attack occurred.
The men -- Usama, 31, a driver, and Mohie, 27, a plumber -- then put Iman on the lower floor of the building, beneath stairs, and said she had fallen to her death, the rights group said.
The Giza criminal coroner found marks on the girl and informed Giza security.
Iman's younger sister told authorities her uncles had killed her sister.
After being arrested and confronted with the younger sister's story, the men allegedly confessed to beating Iman, saying they were trying to help the girl's mother change her daughter's behavior, authorities said.