His employer told Esther Lubisi and the police that her brother, David "Dayk" Lubisi, was last seen walking into a crocodile-infested river.
Police are investigating the mysterious circumstances under which Lubisi disappeared.
The employer told police that a neighbouring farmer had reported seeing a crocodile with a human leg protruding from its mouth in the river formerly known as Olifants river last Monday.
He further told them that Lubisi was drunk when the incident happened on the evening of Thursday, April 7.
But Esther says it is "impossible" because Lubisi did not drink alcohol at all He was a member of the Zion Christian Church and adhered to the church's doctrine.
"I think the police must look at this very closely. My brother was a sober man. I insist that he did not drink alcohol."
Lubisi, 40, of Tsuvulani village near Bushbuckridge, Mpumalanaga, had worked on the farm outside Phalaborwa for just more than five years.
"I think my brother was killed and fed to crocodiles, or just thrown into the river," Lubisi said. "I have asked police to be thorough with their investigation.
"There are many unanswered questions. His employer did not immediately come to us or send one of the other employees to come and tell us about the incident.
"We only got to know about it when police came to the family two days later."
Jesper Kehlet, owner of Balule River Camp, said he had gathered from his employees that Lubisi had thrown himself into the river.
He said Lubisi had drunk alcohol on two previous occasions.
But Kehlet contradicted himself when he said he would "immediately" fire any employee found drinking at work.
The farmer claimed he was in the office when Lubisi, allegedly one of his trusted workers, went into the river.
Kehlet, a Danish national, said he phoned the Phala-borwa police as soon as he was notified of Lubisi's disappearance.
Phalaborwa police commander Colonel Masilo Selepe confirmed that he went to the scene shortly after receiving a call from the farm on April 7, but he would not specifically say who the call came from.
"We are currently investigating the circumstances that led to the disappearance of a man in a river," Selepe said yesterday.
Lizah Lekhuleni, a ward councillor in the Bushbuckridge municipality, called on the government to tighten laws to stop the murder of farmworkers "because the practice is becoming prevalent".
Lekhuleni said the municipality would support the family of Lubisi until the truth came out.