Darren was killed in his white Nissan, which was linked to the scene of the robbery. He was shot nine times, reportedly by a single officer.
Yolandi said police claimed he reached for one of two "BB guns" on the passenger seat, but she questioned the extent of the officer's response.
"Why did they have to shoot him nine times?" she asked. "He was a harmless boy."
Moses Dlamini, spokesman for the Independent Police Investigative Directorate, said an investigation was under way.
No arrests have been made.
As Yolandi prepares for Darren's memorial service on Tuesday, she said "the family will miss him tremendously, he was their whole life".