The teenaged girl, identified as Khushboo Kumari (13) cooked vegetables laced with a poisonous substance for dinner for her father, Shankar Mahto (35) and brother, Vikas Kumar (12) last night, police said.
The father and son died in sleep after consuming the poisonous stuff. Mahto's wife, who did not have dinner as she was feeling unwell, got reprieve, police said.
The bodies have been sent for post-mortem.
Kumari confessed to her crime and has been taken into custody on the charge of murder of her father and brother, the police said adding her boyfriend Vijay Kumar and his mother Shanti Devi, who have been made as accused in the case by the girl's mother, Sunita Devi, were absconding.
The girl had earlier tried to elope with her lover but was apprehended by the villagers, Kumar said.