The 27-year-old double amputee shot dead the South African model by firing four gunshots through a toilet door at his home in the early hours of Valentine's Day last year.
Last week, Pistorius's defence team called for Judge Thokozile Masipa to sentence the athlete to three years house arrest and community service, while the prosecution believe that at least 10 years behind bars would suffice.
In an interview with the Mail Online, Ms Cowburn said: "Oscar deserves prison. How can he just stay in the luxury of his house after taking somebody's life. What is house arrest if not just wrong?
"Community service and house arrest are ridiculous. These should be for people who are stealing or other minor offences. Not shooting somebody. Reeva deserves justice."
Pistorius claimed that he mistook Steenkamp for an intruder when he fired the gun.