President Jacob Zuma has updated SA on Madiba's condition
Ndaba said his family was grateful for Madiba's life and they were not ready to contemplate his death, the Star reported on Friday.
"Only God can take him… but for us, as family, as long as he can still hear and understand what is said to him, and talk to us, we'll continue to celebrate him."
Mandela was admitted to Pretoria Medi-Clinic Heart Hospital for a recurring lung infection on June 8.
Ndaba thanked people around the world for sending well-wishes to his grandfather.
For the first time, members of the Mandela family on Monday gathered well-wishes from outside the Pretoria hospital where Mandela is being treated.
Family members at the time braved the media encampment outside the Pretoria hospital where he is being treated to gather messages of support that have been posted outside a vehicle entrance.
Madiba is "doing very well", his daughter Zenani said during the brief outing at the start of the week.
Mandela's wife Graça Machel on Monday morning issued a statement in which she expressed gratitude for the outpouring of support for Mandela, but provided no details on his state of health.
"So much love and generosity from South Africans, Africans across the continent, and thousands more from across the world, have come our way to lighten the burden of anxiety, bringing us love, comfort and hope," Machel said. – Sapa; additional reporting by Phillip de Wet