Kim Martin said she had "hoped to God" that Pistorius was cheating on her cousin and that the young woman he had shot at his home was not Reeva.
"They hadn't confirmed the name, they said his girlfriend," Martin recalled of the early-morning news broadcast. "I was trying to phone her (Reeva) and she wasn't answering and I was screaming at my husband."
Martin, who said she was close to Steenkamp, went to her mother's house where she learned her cousin had been killed by the Olympic athlete.
"The doors opened and my mother was hysterical and that's when I knew it was true," Martin said. "That was for me the end of the world. Everything was just a blur from then onwards."
The emotional testimony during Pistorius' sentencing hearing caused the judge to rule a brief adjournment when Martin began crying. Her testimony dealt in detail for the first time at the months-long trial with the life of Steenkamp, a 29-year-old model and law graduate, and the impact her Valentine's Day killing had on her family.
"We were all, 'why, why, why Reeva?'" Martin said. She said Steenkamp's killing "ruined our whole family."
Steenkamp was shot multiple times by Pistorius through a toilet cubicle door in his home in the pre-dawn hours of Feb. 14, 2013. The runner was found guilty of culpable homicide for negligently killing her, but acquitted of murder after claiming he mistook Steenkamp for an intruder.
Martin was the first witness called by prosecutors for sentencing and the first relative of Steenkamp to give testimony in the case. Martin said she wanted to be "Reeva's voice" after Steenkamp's parents, Barry and June, decided they were too emotional to testify.
As Martin told of the moments she first learned Steenkamp was dead, Pistorius hunched forward in the courtroom and put his hands over his face. Steenkamps' parents, who have attended much of the trial, appeared to be crying.
Judge Thokozile Masipa will decide Pistorius' punishment and has wide latitude. The judge could order the former star athlete to pay a fine and go under house arrest, or she could send him to prison for up to 15 years.
Pistorius' lawyers have argued for a correctional supervision sentence with periods of house arrest. Social workers testifying for the defence recommended he not be sent to prison, partly because of his disability as a double amputee. Prosecutors reject that and insist that Pistorius, 27, should go to jail, saying his negligent actions on the night he shot Steenkamp with his 9 mm pistol have left a "broken family."
Steenkamp's mother June was on medication after the killing and father Barry sat in a corner of his house crying, Martin testified. Martin said that she is receiving trauma counselling and her children need therapy.
Steenkamp's parents said in a statement earlier Wednesday that the death of their daughter — who had helped them with money — had left them in financial difficulties and their lawyer resorted to approaching Pistorius for help. The athlete had paid them around $10,000 in monthly instalments between March 2013 and last month, but they now want to pay it back, they said.
The Steenkamps also said they turned down a separate settlement offer of $34,000 from Pistorius and would no longer pursue a civil claim after the criminal trial because they didn't want his money.
Martin testified that Steenkamp's parents were never rich, but father Barry was particularly proud of his daughter's law degree and wanted her to concentrate on becoming a lawyer.
Martin said: "Her dad told her she must work hard because her life can end in a split second."
Imray reported from Stellenbosch, South Africa.