A 13-year-old Brazilian girl has murdered her pregnant sister and her son before tearing the unborn baby from the womb on the orders of a 35-year-old woman who wanted to keep the baby for herself.

Fabiana Santana, 23, was eight months pregnant when the girl attacked her, Dailymail reports. Santana’s seven-year-old son Gustavo was then drowned when he tried to save her in Porto Velho, Brazil.

The pregnant sister was bludgeoned around the head with an iron bar, leaving her face disfigured and stabbed to death.

Police say the 13-year-old girl carried out the attack with a 15-year-old boy on the orders of Catia Rabelo who masterminded the plan. She was arrested on Wednesday after going on the run and has allegedly confessed to inciting the teenagers to commit the brutal killings.

It is believed Rabelo intended to trick her boyfriend, a gold prospector, into believing she had given birth to his child. Remarkably, police found the baby alive in the care of the 15-year-old accomplice at his family’s home.

The 15-year-old’s mother, who was not present at the murders, had fled from the home. The child, weighing 3.9lbs, is under observation at Porto Base Old Hospital and is in good health despite his horrifically violent birth.

The expectant mother and her son were reported missing by relatives after they failed to return from an evening out on October 18 with the teenage suspects.

Mrs Santana body was found buried in a shallow grave on a vacant plot of land in Porto Velho three days later.

An autopsy report released on Friday suggests she may have been alive when her baby was cut from her womb with a knife. Her young son was found floating in a man-made lake, less than 100 metres from where his mother was buried.

Investigators are also trying to substantiate allegations made by the adolescent girl against her deceased sister and a family member.

Police chief, Leisaloma Carvalho told FocusOn News: ‘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.’