Aimee Pistorius watches as her brother, South African Olympic and Paralympic track star Oscar Pistorius, attends his sentencing at the North Gauteng High Court in Pretoria October 21, 2014
Aimee Pistorius watches as her brother, South African Olympic and Paralympic track star Oscar Pistorius, attends his sentencing at the North Gauteng High Court in Pretoria October 21, 2014Reuters

Oscar Pistorius' sister has hit back at claims that the convicted killer celebrated his birthday with a cake, balloons and a gift bag, after his siblings Aimee and Carl visited the "Blade Runner" in prison when he turned 28 years old on 22 November.

Aimee Pistorius debunked the "false and irresponsible" reports in a series of tweets and denied that her brother had received preferential treatments during his birthday.

Pistorius is one month into a five-year prison sentence for shooting dead his 29-year-old model girlfriend Reeva Steenkamp at their Pretoria home on Valentine's Day last year.

He was also found guilty of culpable homicide and two firearms charges at an earlier hearing.

Judge Thokozile Masipa cleared the athlete, known as the Blade Runner, of murder after the prosecution was unable to prove he intended to kill Steenkamp.

Throughout the eight-month trial at the High Court in Pretoria, prosecutor Gerrie Nel vigorously argued that Pistorius had intended to kill his girlfriend.

Meanwhile, South Africa's Department of Correctional Services said it would investigate the claims over Pistorius receiving preferential treatment on his birthday and the information given to the media.