(CNN) -- South Africa sent condolences to Nelson Mandela on Monday following the death of his youngest sister and last surviving sibling out of 31 others.
Nokuthamba Mandela, 81, died Saturday, South African President Jacob Zuma said.
The cause of death was unclear.
"On behalf of myself, the people and government of South Africa, I would like to convey our heartfelt condolences to Madiba and the entire Mandela family at this difficult time," Zuma said in a statement. "Our thoughts and prayers go out to you in this time of sorrow."
The former president -- affectionately known by his clan name Madiba -- last appeared in public in the closing ceremonies of the 2010 World Cup in South Africa.
His sister's death came the same weekend he returned to Johannesburg while his home in Qunu undergoes repairs. Mandela, 93, has been living at the house in the Eastern Cape province since last year.
Despite his rare appearances following a respiratory infection, Mandela retains his popularity and is considered the founding father and hero of South Africa's democracy.
Under South Africa's apartheid regime, Mandela spent 27 years in prison after being convicted of sabotage and attempts to overthrow the government.
He was released in 1990 and became president in 1994.