Pistorius claims he shot Steenkamp, a 29-year-old law graduate and model, four times through a locked bathroom door at his upmarket Pretoria home after he mistakenly believed she was an intruder.
His lawyers, arguing that Pistorius would be more vulnerable than most in prison because of his disability, have called for three years of "correctional supervision", the equivalent of house arrest for the 27-year-old.
They also suggested that Pistorius could carry out 16 hours of community service a month cleaning a Pretoria museum.
Prosecutor Gerrie Nel said such a sentence would be "shockingly inappropriate" and could cause South Africans to lose faith in their legal system.
On the eve of the sentencing at the Pretoria High Court, Pistorius's siblings Aimee and Carl spoke to the media for the first time since the fatal shooting.
They vowed to stand by Pistorius as he faces his punishment and said the family had struggled to cope with the death of Steenkamp, of whom they had become fond in the few months the couple were together.
"It's important for us that they (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," said Aimee, who has sat through each court session since the trial opened in March.
"It's been difficult to try support someone through this grief," she told local tv station eNCA.
"The guilt and ridicule that surrounds it as well as the exposure that has come with it, and just the heartache for both my brother, my family and of course the Steenkamp family."
Pistorius would at times weep and vomit during the high-profile trial, large chunks of which were broadcast live around the world, including on a specially-created 24-hour television channel.
Pistorius's older brother Carl said his family would "stand strong" on Tuesday but refused to speculate on the sentencing outcome.
"I don't think one can ever be prepared for whatever the sentence might be," he said.
Legal experts were divided on which way Judge Masipa will swing.
"There is a strong argument to be made for certainly a period of direct imprisonment," said William Booth, a criminal lawyer based in Cape Town.
"You do have to send a message to the public."
Despite being unpalatable to many South Africans, a house arrest sentence would be fitting for Pistorius, said Kelly Phelps, a law lecturer at the University of Cape Town.
Defence lawyer Barry Roux said the "broken" Pistorius was an ideal candidate for a non-custodial sentence given his remorse, his status as a first-time offender and the fact that he would be an easy target in South Africa's notoriously brutal jails.
The athlete made history by becoming the first Paralympian to compete against able-bodied athletes at the 2012 London Olympics, inspiring millions with his story.
But during his trial the prosecution painted a darker picture of the one-time sports star, presenting a dangerously volatile young man with a penchant for guns, beautiful women and fast cars.
Both the state and the defence have the right to appeal the verdict, potentially dragging out the legal proceedings for years to come.
- AAP
