A 13-year-old Brazilian girl has been arrested over the murder of her pregnant sister who she is accused of killing before cutting out her baby and stoning another child to death.

Fabiana Santana, 23, was eight months pregnant when she was bludgeoned with a metal pole and stabbed to death by her little sister, The Sun reported.

Her 7-year-old son Gustavo was also murdered when he tried to save her in Porto Velho, Brazil.

It is alleged that the girl was aided by a 15-year-old boyfriend.

Reports said that the boyfriend was the son of Catia Rabelo, 35, and the alleged mastermind behind the barbaric murders.

Fabiana and Gustavo were reported missing by family when they failed to return from an evening out on Oct. 18, with the suspect.

Santana’s body was discovered in a shallow grave three days later and Gustavo’s body was discovered floating in a lake less than 330 feet away from his mother’s body.

Her face was left disfigured. An autopsy carried out on her body found that it is likely that she was actually still alive when the baby was cut from her womb.

Rabelo went on the run but was arrested on Wednesday, Oct. 23, and is said to have confessed to inciting the teenagers to commit the killings.

Police found the newborn boy alive and in the care by the boyfriend’s family.

The newborn, which weighed just under 4 pounds, was taken into care at Porto Base Old Hospital where he is said to be in good health.

The girl suspect has reportedly admitted to killing her sister and nephew.

“The girl made a separate claim that she committed the crimes out of revenge because she was sexually abused by her sister’s husband and ill-treated by Fabiana who didn’t believe her stories,” Police Chief Leisaloma Carvalho said.

Police said Rabelo’s motive is believed to have been a desire to trick her boyfriend into believing she had delivered his baby so her boyfriend would marry her.