Durgadevi, along with her two brothers, killed the girl and dumped the body in a nearby hut on Saturday, they said, adding the body was recovered by villagers and police were informed.
Police have arrested the mother, who during the interrogation, confessed to killing the girl as she suspected her of having illicit relations with three persons, SP, Ballia, Manoj Kumar Jha said.
An FIR has been registered against the three, he said.