Mandla Mandela, 38, was accused of bigamy when he married teenager Anais Grimaud while still married to his first wife.
Now, following weeks of media speculation that their marriage had ended amid blazing rows, Mr Mandela has confirmed that his wife had an affair with his brother.
“The Mandela family has sent my wife Nkosikazi Nobubele Mandela (nee Anais Grimaud) back to her home after it was discovered she has been having an affair with one of my brothers”, he said.
“I confirm this affair resulted in a son that Nkosikazi Nobubele gave birth to in 2011.
The cuckold scandal is just the latest in a series of controversies involving Mandla Mandela, whom Nelson Mandela anointed as his political heir.
As well as being a South African MP, Mandla Mandela is a chief in the important Thembu tribe and handles much of his grandfather’s legacy.
As grandson of the universally respected 94-year-old he is also the public face of the Mandela family.
He married his first wife, Tanda Mabunu-Mandela, at a traditional ceremony in 2004.
Six years later he married Miss Grimaud, who is from the island of Reunion, a French overseas territory in the Indian Ocean.
On marriage she took the tribal name Nkosikazi Nobubele Mandela.
The couple’s son was born in September last year and was named Qheya by Nelson Mandela himself.
The marriage was marred by the fact Mandla remained officially married to his first wife.
Ms Mabunu-Mandela fought the Grimaud marriage in the law courts and it was last year declared unlawful.
In a statement released this week, Mandla Mandela said DNA tests confirmed that the boy was not his son but he did not identify which of his two brothers was responsible.
“The revelation of this affair has come as a shock to me and the rest of my family,” the statement reads.
“It has been made more painful because it is my own brother who is at the centre of the crisis.”
Earlier this year Mandla Mandela was forced to deny selling exclusive rights to film the anti-apartheid icon’s funeral for around £250,000.
Mandla’s first wife Thando claimed that the tribal chieftain was alleged to have cut a deal with the South African Broadcasting Corporation in conjunction with the BBC.
He had also attracted criticism for ordering that the bodies of three of the former president’s children be exhumed from their home village of Qunu and reburied in the nearby hamlet of Mvezo where he is a chief.
Mandla’s motive, said his critics, was to ensure that the former president be buried in the remote Eastern Cape town: something that is sure to act as a lure for big-spending tourists.