The 26-year-old double amputee and six-time Paralympic sprint champion will appear in court in Pretoria charged with the premeditated murder of 29-year-old Reeva Steenkamp in February.
The South African athlete, who is expected to stand trial later this year or early next year, is on bail after a judge changed his conditions in March.
The appearance comes after Ms Steenkamp's parents Barry and June gave a television interview saying they were desperate to find out why their daughter had been shot.
In the Channel 5 interview, aired Monday, Mrs Steenkamp said "there is only one person who knows what happened" that night in February when her "terrified" daughter was killed .
Pistorius's family said they will "support him every step of the way" and "fully stand behind" him.
The Blade Runner's uncle Arnold Pistorius said the family were "shaken" by pictures of the star's bloody bathroom that emerged last week.
He said: "We believe in him, love him and will support him every step of the way in what lies ahead.
"We were shaken by the graphic images, leaked into the public domain this week, of the accident scene at Oscar's house.
"It has always been our plea that the legal process be allowed to run its course with integrity. The leaking of evidential material into the public domain before the court case does not advance this process.
"We continue to have great faith in the South African legal justice system and believe that Oscar's account of what happened on that terrible night in February will be borne out by the evidence that the defence team will lead in court."
In the interview, Mrs Steenkamp spoke of the guilt she and her husband feel about not having "protected" their daughter.
"Why? Why did he shoot her? I want to know why he shot her. Because she must have been so afraid in the toilet, and somebody's firing (a) gun, bullets through the door."
She went on: "We feel bad that we couldn't protect her. Her whole life we protected her. From the day she was born we protected her, but this we could not protect her from. Why couldn't we protect her from this?
"How did she end up with a person like this? Who could shoot her? I want to know. We don't know what happened. There is only one person who knows what happened," she told the Channel 5 programme Why Did Oscar Pistorius Kill Our Daughter?
Ms Steenkamp's parents said their daughter told them of her arguments with Pistorius and how they had feared for her safety.
Recalling a phone call from her daughter who was travelling in a car that Pistorius was driving at speed, Mrs Steenkamp said: "She was afraid, she was so afraid. She phoned me and she said, 'Mummy I'm in the car with Oscar and he's driving like a lunatic'."
Mrs Steenkamp added: "A week or so later she phoned me, we chatted about this and that, little girl things. I said 'how's it going with Oscar?'. She said that 'We've been fighting; we've been fighting a lot'.
"She didn't elaborate about what they'd been fighting about. She said, 'We are fighting a lot'. That's what she said.
"I didn't feel alarmed about that because men and women do fight, don't they, it's part of a relationship, but this is a very early relationship to be fighting."
Ms Steenkamp's parents also revealed unseen personal photographs and letters.
Among them was a picture painted by her as a teenager, depicting a gunman, an angel and a stairway to heaven which her parents described as a "premonition".
The documentary showed the scene as friends and family came together to scatter Ms Steenkamp's ashes in the waters of Nelson Mandela Bay.
Pistorius will appear in court at 9am local time when the case is expected to be postponed until August.