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 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.
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"We were all, 'why, why, why Reeva?'" Martin said. She said Steenkamp's killing "ruined our whole family."
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.
Pistorius' lawyers have argued for a correctional supervision sentence with periods of house arrest. Social workers testifying for the defense recommended he not be sent to prison, partly because of his disability as a double amputee.