Barry Steenkamp said the Paralympian will "suffer alone" if his claim that he accidentally shot Reeva Steenkamp, 29, is false.
But Steenkamp told the Beeld newspaper he might be able to forgive Pistorius, below, one day if the double-amputee is telling the truth. His remarks came as the athlete appeared to issue his first public statement after being released on bail on Friday, charged with premeditated murder over the killing of Miss Steenkamp.
In a tweet apparently signed by the athlete, using his elder brother Carl's Twitter account, he said: "Thank-you to every person that has prayed for both families, Osca."
Pistorius, 26, known as "Blade Runner", was granted bail following a four-day hearing, a week after he shot Steenkamp dead at his home.
The athlete, who broke down in tears as chief magistrate Desmond Nair announced his decision, was later seen arriving at his uncle Arnold's home.
Pistorius claims he shot Steenkamp through a bathroom door thinking she was an intruder, but prosecutors say he intended to kill the model.
After a lengthy summation, Nair told the packed courtroom that he was granting bail until June 4.
He said there was no suggestion Pistorius was a flight risk, he did not appear to have a propensity to violence, and there was no evidence that he would interfere with witnesses.
Nair said Pistorius had submitted a sworn affidavit describing what happened that night, telling the court: "I come to the conclusion that the accused has made a case to be released on bail."
Bail was fixed at one million rand (£73,822), including 100,000 rand (£7382) in cash. Pistorius must surrender any firearms and his passport, cannot enter any international departure hall, and is banned from using any prohibited substance or alcohol.
He must also report to police twice a week.