The news comes just a day after the demise of Celine's husband Rene Angelil, who passed away due to throat cancer.
Daniel may only have a few days to live, says sister Claudette Dion, reported Aceshowbiz.
Claudette said her brother's illness was similar to Rene's. He is surrounded by family, including their 89-year-old mother. Celine's rep has confirmed reports about Daniel's prognosis.
As of now, the "My Heart Will Go On" hitmaker is preparing for Rene's funeral. Celine wrote in a statement posted on her official Facebook that Rene's funeral would be held at a place in accordance with his final wishes.
According to the Facebook post, a Chapel of Rest will be open to the public on January 21 followed by a religious service on January 22.
A public memorial in Las Vegas, where Celine is now on an extended residency, is scheduled on February 3 at the Caesars Palace Colosseum.