Kim Martin, the cousin of Oscar Pistorius' deceased girlfriend Reeva Steenkamp, began testifying at Pistorius' sentencing hearing at the High Court in Pretoria on Wednesday (local time).
She told the court the family was struggling to cope with Ms Steenkamp's death.
"The scars, the effects runs very, very deep. We'll never completely get over it," said Ms Martin.
The double-amputee Olympic runner's hearing started on Monday.
It will be the second time Pistorius has been sentenced for the killing following an appeal by prosecutors.
Pistorius was convicted of murder by South Africa's Supreme Court of Appeal for shooting Ms Steenkamp on Valentine's Day in 2013.
The 29-year-old gold medallist faces a minimum 15-year jail term after his manslaughter conviction for the 2013 killing, for which he originally received a five-year sentence, was upgraded on appeal.
South Africa has a minimum sentence of 15 years in prison for murder, although a judge can reduce that in some circumstances.
"As far as I am concerned Barry [Ms Steenkamp's father] is a broken man," Ms Martin told the court on Wednesday. "He lives day to day, on his phone, just watching the posts about Reeva."
On Tuesday, Ms Steenkamp's father told the court Pistorius must pay for murdering her, adding that forgiving the Paralympian runner for his crime was very hard.
Called to testify by the lead state prosecutor in Pistorius' sentencing hearing, a tearful and trembling Mr Steenkamp said his daughter's death had devastated her family.
"I ended up having a stroke and so many things since then have happened ... I just don't wish that to anybody in this world," he told the court.
Ms Steenkamp said he and wife, June, had relied financially on their daughter, and he had "jabbed [himself] ... with needles" to try to relive the pain that she went through.
Prosecutor Gerrie Nel told the court on Monday, the first day of the hearing, that Pistorius has shown no remorse for shooting and killing Ms Steenkamp when he fired four shots through a locked toilet door in his Pretoria home.
Mr Nel disputed a claim by Jonathan Scholtz, a psychologist called by Pistorius' lawyer, that Pistorius was "a broken man" who should be hospitalised and not jailed, saying the track star had been violent while serving his sentence.
The case has prompted a fierce debate in a country beset by high levels of violent crime against women. Some rights groups have said the white athlete has received referential treatment.
Earlier on Tuesday Ebba Gudny Gudmundsdottir, from Iceland, described the athlete as an inspiration to her 11-year-old son, who has a similar disability to Pistorius.
Newshub. / Reuters