JOHANNESBURG (REUTERS) - The sister of South African Paralympic star Oscar Pistorius, who was sent back to jail for six years on Wednesday in what many viewed as a lenient sentence for murdering his girlfriend, said on Sunday that she was "grateful" to the judge.

Judge Thokozile Masipa has been criticised by women's and other rights groups for accepting the defence argument that a lesser sentence than the 15-year minimum term sought by prosecutors was appropriate for the 2013 murder of Reeva Steenkamp.

"The thing I'm most grateful (for) and the thing I really appreciate is the emphasis she made to distinguish a difference between the facts and the truth in terms of this is not a gender-based violence situation," Aimee Pistorius said in an interview with the eNCA television news channel.

"It's a terrible accident where Oscar had no intention of shooting Reeva," she said.

Paralympic gold medallist Pistorious fatally shot his girlfriend four times through a toilet door at his Pretoria home and has already served 12 months in prison, but the original manslaughter conviction was increased to murder by the Supreme Court of Appeal in December.

Pistorius says he fired four shots into the toilet on Valentine's Day 2013 in the mistaken belief that an intruder was hiding behind it.

The track star, who had the lower part of his legs amputated when he was a baby, was freed from prison last October after almost a year behind bars.

He was to serve the remainder of the original five-year term under house arrest at his uncle's house in a wealthy suburb of the South Africa capital.