“Oscar, broken as he is, believes he has a purpose in life. Reports to the contrary are untrue,” his uncle said.
Mr Pistorius denies the charges of premeditated murder.
The Paralympian, on trial for shooting model Reeva Steenkamp at his home in Pretoria last month, was released on bail on 22 February. He is due to appear in court again in June.
Mike Azzie, a family friend who has been in regular contact with the athlete, told the BBC he was worried about his mental and emotional state.
He made the comments as part of a forthcoming BBC3 documentary, Oscar Pistorius: What Really Happened?
“I would say that, just speaking to him, he is a broken man. I would go as far as to say that he could be on the verge of suicide. It really worries me,” Mr Azzie said in the interview, to be aired on Monday evening.
‘Terrible and tragic’
But Mr Pistorius’s uncle has rejected Mr Azzie’s observation.
“Of course Oscar will never be the same, having to live with the knowledge that he caused the death of the woman he loved, but he is coping as best he can with the support of his loved ones,” Arnold Pistorius said in a statement on Monday afternoon.
“As a family, we remain confident that this will prove to have been a terrible and tragic accident, which has changed many lives forever.”
Although the family did not deny that Mr Pistorius had sold some of his belongings to cover rising legal costs, his uncle said the athlete would evaluate the cost situation on a day-to-day basis.
“We are acutely aware of the fact that this is only the beginning of a long road to prove that, as we believe, Oscar never intended to harm Reeva Steenkamp. We also realise that the law must run its course, and we respect this process.”
In a separate development, defence lawyers have sought to ease bail restrictions imposed on Mr Pistorius.
The court had ordered the athlete to hand over his passport, avoid his home in Pretoria and report to a police station between 07:00 and 13:00 every Monday and Friday.
But his lawyers said the conditions were “unwarranted and not substantiated by the facts”.
Evidence presented at the trial showed he was not a flight risk, and should be allowed to travel outside South Africa with official permission, they argued.
Ms Steenkamp, 29, died after Mr Pistorius fired multiple shots into the toilet of his apartment in a gated housing complex in Pretoria on 14 February.
Mr Pistorius says he mistook his girlfriend for an intruder.