"He is doing well physically, but he has been suffering from dementia for a long time," Jaime Garcia Marquez was quoted as saying by BBC.
"He has problems with his memory. Sometimes I cry because I feel like I'm losing him," he said.
The 85-year old Colombian novelist, affectionately known in Colombia as Gabo was awarded the Nobel Prize in Literature in 1982.
Jaime, his younger brother, is the first family member to speak publicly about it.
"He still has the humour, joy and enthusiasm that he has always had."
Marquez, a central figure in the magical realism movement in Latin American literature, 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," Jaime was quoted as saying by The Guardian.
He 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.
Best known for 'One Hundred Years of Solitude' which went on to sell more than 30 million copies and was translated into 37 languages, Marquez currently lives in Mexico and has not made many public appearances in recent years.
His last novel, Memoirs of My Melancholy Whores was published five years ago.
