Jaime García Márquez told students in Cartagena, Colombia, that his older brother, affectionately know as Gabo, calls him on the telephone to ask basic questions.
"He has problems with his memory. Sometimes I cry because I feel like I'm losing him," he said.
The 85-year-old Colombian writer won the Nobel prize in 1982 and is best known for novels including One Hundred Years of Solitude, Love in the Time of Cholera and Chronicle of a Death Foretold.
He has fought a long battle against lymphatic cancer which he contracted in 1999 and it is believed that the cancer treatment has accelerated his mental decline.
"Dementia runs in our family and he's now suffering the ravages prematurely due to the cancer that put him almost on the verge of death," said Jaime. "Chemotherapy saved his life, but it also destroyed many neurons, many defences and cells, and accelerated the process. But he still has the humour, joy and enthusiasm that he has always had."
Jaime said that he tried to keep his brother's condition a secret, "because it's his life and he's always tried to protect it". However, he was moved to speak openly because of the inaccurate speculation he encountered.
Jaime said: "The fact is there are lots of comments. Some are true but they're always filled with morbid details. Sometimes you get the sense they'd rather he were dead, as if his death were some great news."
Márquez's chaotic upbringing provided ample material for his complex novels.
His parents' courtship was resisted by his maternal grandfather, who objected to his father's conservative political views. The wedding was eventually allowed to take place and Márquez was born in Aracataca which became the fictional village of Macondo in his novels.
Márquez's parents left him in the care of his maternal grandparents when they travelled to work.
His grandfather was a soldier and a hero of the liberal movement and instilled in him an interest in social justice and the gravity of taking human life.
Márquez now lives in Mexico and has not written anything since his last novel.
The popularity of his writing led to friendships with many prominent Latin Americans including the former Cuban leader Fidel Castro.
Jaime García Márquez, who heads the Ibero-American New Journalism Foundation, founded by Gabo in 1994 in Cartagena, said it was regrettable that his brother was not in a condition to write the second part of his autobiography, Living to Tell the Tale.
"Unfortunately, I don't think that'll be possible, but I hope I'm wrong," he said.