Mildred Mthethwa said a man arrived at their home carrying a corpse and asked if someone could identify it, African Eye News Service reported on Tuesday.
"It was my brother," she said.
Mthethwa said the man found Bethwa attended a secret African National Congress meeting on a mountain in Kanyamazane.
She said a group of youths sang freedom songs on the way home and were attacked by police and members of the Kabasa gang.
She said the stranger had gone to several houses with the corpse trying to find his family.
Her father phoned undertakers who arrived with policemen, she said.
"When the police kicked my brother's body, my family armed themselves with anything they could find in the house."
She said the driver of the hearse was terrified and would not put her brother's corpse in the vehicle.
"My father said he was prepared to carry my brother all the way to the mortuary, but the hearse driver wouldn't let him and agreed to take Bethuel," she said.
Mthethwa said her family had always warned Bethuel not to get involved in politics. "But he said if he would die, it would benefit the community."
Mthethwa asked the commission for a tombstone to commemorate her brother.
"Dead comrades were always given tombstones to commemorate them, but my brother wasn't," she said.