Patna, May 24 : With the help of her boyfriend, a 13-year-old girl in Bihar's Vaishali district killed her younger sister, a police official said. The incident took place in Jandaha block and the two accused were arrested on Tuesday. After committing the crime, the two had kept the nine-year-old's body in a box inside the girl's house. As the body began to rot emanating a foul smell, they then chopped the body, burnt it with acid and threw it in the backyard of the house. Vaishali SP Ravi Ranjan Kumar said the incident came to light only after some villagers spotted the remains and informed the local police. "We then sent a dog squad and FSL team to the crime scene. The team collected the remains and investigations revealed that it belonged to a minor. Accordingly, we launched a probe into missing children in area and found that a nine-year-old girl had disappeared on May 16 and her remains were recovered on May 19. We then sent the evidence to the lab and an autopsy was also conducted. Once we had enough proof, we arrested the two accused on Tuesday evening," Kumar said. "When we questioned the victim's elder sister, she broke down and revealed the entire sequence of incidents. The accused said that she and her boyfriend were intimate a few days ago and her sister saw them in a compromising position. She then decided to kill her sister." The girls' father is a daily wage labourer and was not present during the time of the incident. /IANS