Aimee and Carl Pistorius said they were anxious ahead of Tuesday's hearing, and that the truth had been "twisted and manipulated" and "sensationalised".
Pistorius was convicted of culpable homicide after shooting dead girlfriend Reeva Steenkamp, but cleared of murder.
The prosecution has argued for a minimum 10-year jail sentence.
Speaking to South African broadcaster eNCA, Aimee Pistorius said there was "obviously a lot of anxiety" ahead of the sentencing, and that the last 20 months had been "very taxing".
She said she felt "heartache for both my brother, our family and of course Steenkamp's family and her friends", adding that both families would struggle to move on.
"It's something that you never really get closure on... It's something that my brother will carry with him for the rest of his life.
"It is important for us that [Ms Steenkamp's family] know that she was very much cared for and loved, and accepted as part of our family in the short time that she was with us... [Reeva] had a wonderful charisma and zest for life that she just exuded and it was easy to just fall in love with her."
She added: "The truths had been twisted and manipulated, and things have been sensationalised. It causes hurt when things are also reported as fact when they are not."
Carl Pistorius said the hardest part of the situation was "all the negative media, polarised publicity, twisted truths and untruths, and how that infiltrates your life regardless of whether you try to or try not to follow the media".
"It permeates the very atmosphere of everywhere you go, including those that are close to you," he said.
He added that he could not speculate on the outcome of the sentencing, but said "tomorrow will also be very difficult. This is a weight that we all carry".
The high-profile, seven-month trial has captured public attention in South Africa and beyond.
Pistorius, a Paralympic sprinter, denied murdering Ms Steenkamp on Valentine's Day last year, saying he shot her by mistake, fearing there was an intruder in the house.
Ms Steenkamp, a model, reality TV star and law graduate, was hit three times by bullets shot through a toilet door by Pistorius at his home in the capital, Pretoria.
Last month, Judge Thokozile Masipa cleared Pistorius of murder, but convicted him of culpable homicide, saying he had been negligent.
There is no legal limit on the length of a jail term, but experts say the typical maximum sentence for the crime is around 15 years.
Pistorius' defence team has argued for him to be given community service and house arrest, a suggestion the prosecution said would be "shockingly disproportionate".
Oscar Pistorius' siblings have criticised media coverage of his trial, on the eve of the South African athlete's sentencing.