Their family has been accused of crass commercialism, exploiting their grandfather's name and tarnishing his legacy by fighting over his final resting place. Undaunted, two of Nelson Mandela's granddaughters chose the eve of his 95th birthday to launch a collection of 299 rand (£20) T-shirts emblazoned with "Legend", "Revolutionary" and "Long Walk to Freedom".

Swati Dlamini and Zaziwe Dlamini-Manaway said they would be among 30 to 40 family members visiting the former president in hospital in Pretoria on Thursday. They said he could expect 95 cupcakes along with cards made by their children, and described his condition as "still critical, but he's doing much better".

The pair, who featured in the critically mauled reality TV show Being Mandela, had posted half a dozen tweets on their joint Twitter account inviting allcomers to the launch in one of South Africa's more luxurious shopping malls. But it could not be described as a public stampede. The handful of shoppers at the Cross Trainer in Sandton were comfortably outnumbered by staff and about 20 journalists eager for health updates.

In a window display there were pictures with fake gold frames showing the anti-apartheid hero. One captured him beaming alongside Michael Jackson.

Swati wore two of the 14 new designs. Asked whether they were competing with the Nelson Mandela Foundation's 46664 brand – a reference to his prison number on Robben Island – she replied: "We are all trying to achieve the same objective."

She added: "We launched it around my grandfather's birthday to celebrate his legacy. My grandfather was also a lover of fashion. He became famous for his Madiba shirts."

Mandela has been described as a global brand comparable with Coca-Cola or Google. Close to where his granddaughters gave a series of interviews, billboards advertised Mandela commemorative coins, souvenir shops offered Nelson Mandela Day mugs and wristbands, bookshops displayed Long Walk to Freedom and other Mandela titles in their windows and tourists posed for photos beside a giant statue in, perhaps inevitably, Nelson Mandela Square.

Swati and Zaziwe, both in their 30s, denied that they were trying to cash in with their Kardashians-style reality show and clothing range. Swati said: "Our grandparents have always told us that this legacy and this name also belongs to us and that we have a birthright to it, and that's essentially it."

Zaziwe said: "This is just us doing us. This is our name as well, there's nothing we can do about it. We were born into this family and it's our name: we love it, we cherish it, we respect it and we are just trying to continue our grandfather's legacy."

When Mandela was admitted to hospital on 8 June with a recurring lung infection and then slipped into a critical condition, many doubted he would see his 95th birthday. But his condition appears to have improved in recent weeks.

In a statement released on Wednesday, his grandson Mandla said: "The fact that my grandfather is with us to this day when doomsday people predicted his demise makes me proud of his fighting spirit which he has demonstrated over the many years of his life."

Mandela's daughter Zindzi said he had made "dramatic progress". She told Sky News: "I visited him yesterday and he was watching television with headphones. He gave us a huge smile and raised his hand … He responds with his eyes and his hands."

Mandela was gaining energy and strength, Zindzi said. "I should think he will be going home any time soon."

Mandela will be among the first recipients of a smart ID card launched by the South African government on Thursday. It will be accepted by Zindzi on his behalf.

His foundation has asked people in South Africa and beyond to mark international Nelson Mandela Day on Thursday by volunteering 67 minutes to charity in honour of his 67 years of public service. President Jacob Zuma will oversee the donation of houses to poor black and white families in "a non-racial settlement" in the Pretoria area, and the retired archbishop Desmond Tutu will help paint a school in Cape Town. Other celebrations in South Africa include the world's biggest birthday card and the world's biggest Mandela portrait.

The country's last white president, FW de Klerk, said: "Nelson Mandela is approaching the end of the long walk that he began 95 years ago in Qunu. On his birthday we should commemorate the remarkable contribution that he has made to South Africa and to all its people. Today should be a time for quiet and respectful contemplation – and not for unseemly squabbling over the ownership of Mr Mandela's heritage."