Battling tears and with her voice trembling, Kim Martin told a sentencing hearing that she was "very fearful" of the South African Paralympic and Olympic sprinter and did not believe his apology for killing her cherished cousin was genuine.
"Pistorius needs to pay for what he has done, for taking Reeva's life, for what he's done to my uncle, to my aunt and the rest of my family," she said.
"Everybody has suffered here, and I really think we need to send a message to society that you can't do this and get away with it."
Pistorius has been found guilty of manslaughter for shooting Steenkamp dead on Valentine's Day 2013.
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He could be sentenced as soon as Friday, but with no mandatory minimum sentence, Judge Thokozile Masipa will have to decide whether he deserves to go to jail or stay free.
They have argued the double amputee would be vulnerable in prison and should receive a community service instead.
Prosecutor Gerrie Nel has slammed that suggestion as "shockingly inappropriate," and warned that "if the court sentence is too light, and society loses trust in the court, they will take the law into their own hands."
The defence has warned that a jail term would "break" the 27-year-old star sprinter -- who inspired millions when he became the first double amputee to compete against able-bodied athletes in an Olympics -- and that he could fall victim to prison violence, including gang rape.