One granddaughter defends Winnie's attack on the ruling party, while another is criticised for speaking at ANC centenary event

A granddaughter of Winnie Madikizela-Mandela has spoken out in defence of the angry criticism of the African National Congress (ANC) made by the stalwart of the South African struggle.

Swati Dlamini endorsed a scathing email, sent by Nelson Mandela's former wife to the ANC, which accused the governing party of disrespecting the Mandela family, the Times of South Africa reported.

"I think that there are some very valid points that she made," Dlamini told the paper at the launch of her clothing label this week. "I won't go into specifics, but you know as her being my grandmother, I support her."

In the email, leaked to the Guardian on Tuesday, Madikizela-Mandela said her family was deeply hurt by its "shabby treatment through the years" at the hands of the ANC. She wrote: "It is quite clear that we do not matter at all, we only do when we have to be used for some agenda."

The 75-year-old "mother of the nation" also made clear that she would be boycotting an ANC centenary lecture about Nelson Mandela that was delivered by President Jacob Zuma in Limpopo province on Tuesday.

Madikizela-Mandela's stand appears to have caused a rift in the Mandela family. While her daughters, Zenani and Zindzi, also stayed away from the event, Nelson Mandela's eldest grandchild, Ndileka Mandela, did attend and made a speech.

Ndileka Mandela's presence at the event has been criticised by supporters of Madikizela-Mandela. A source close to the family told the Guardian: "The Mandela family is of the view that Ndileka Mandela, who attended the lecture and said she is speaking on behalf of the 'entire family', attended in her own capacity and accord, and her presence has no blessing and endorsement by the family and the Nelson Mandela Foundation, and in no way was she representing the family, as she stated."

The source added: "The family reject her lies with contempt. The family wishes to state categorically that it fundamentally sticks to and supports the whole content of the response letter by Mama Winnie to the ANC on the matter."

Contacted on Thursday, Ndileka Mandela confirmed that she believed she was speaking on behalf of the entire family in Limpopo, but she declined to comment further.

Zuma's lecture followed protests and violence as hundreds of supporters of his rival – the expelled ANC youth league president Julius Malema – clashed with police. A 68-year-old man died, reportedly after inhaling pepper spray during the fracas, and five people were arrested.

A day later Zuma visited Mandela for lunch at his home in Qunu, Eastern Cape, before the former president's 94th birthday next week. "It was a delight to see him as always," Zuma said. "I was particularly happy that I had an opportunity to give him good wishes for his upcoming birthday.

"I also informed him that, as usual, all South Africans are waiting for the 18th, preparing to wish him a happy birthday in every possible way."

In her leaked email, Madikizela-Mandela had written: "In the past the ANC never had any interest in celebrating Tata's birthday except to gate crush [sic] on family's arrangements."