|Robin and Maurice Gribb|
The Bee Gees singer lost his long battle with colon and liver cancer on Sunday night and paid tribute to Maurice, who died in 2003 after complications during an operation on a twisted intestine, on his deathbed.
Speaking to family members shortly before he passed away, the 62-year-old star said: "I wish Mo was here, I can't believe he is gone."
His family, including wife, Dwina, brother Barry, daughter Melissa, 37, and sons Spencer, 39, and Robin-John, 29, had been keeping a bedside vigil at a London hospital in recent weeks, and family sources revealed the singer often talked about Maurice during his final few weeks.
But Robin never acknowledged the fact he might be dying, despite drifting in and out of a coma, and he "constantly" spoke of the possibility of touring with his brother Barry and at one point on his deathbed he called out for his son Robin-Junior.
A friend told the Daily Mirror newspaper: "We all knew he was seriously unwell, but his strength of character meant he never once mentioned the possibility he might not make it.
"Instead he talked constantly about touring again, working with his brother Barry and about how much he missed Maurice, or Mo as he always called him.
"It was very moving to hear him speak so lovingly of his twin brother - as though he had never quite accepted that he had gone and still felt the same amazing connection between the two of them that they'd always had.
"He was also an incredibly loving father and husband - on one occasion he woke from his coma and immediately called out for his son, asking 'where's RJ?' "
Barry's older brother Barry, 65, is said to be heartbroken after hearing the news at his home in Miami, Florida, and he made plans to fly back to London with his 92-year-old mother Barbara to mourn with the family.
As well as paying tribute to Maurice, Robin made a dying wish for the Bee Gees classic 1977 hit 'How Deep Is Your Love' to be the first song played at his funeral, asking Spencer to make it possible.
A close friend told The Sun newspaper: "It's a beautiful song, penned by a beautiful person. It couldn't be more appropriate when we all say goodbye."
The group - made up of Robin, his twin brother Maurice and Barry - sold over 220 million records in their career, but they also suffered heartache when their sibling Andy died of heart failure aged just 30.
A number of stars have paid tribute to the late singer including Duran Duran, Liam Gallagher, Stevie Wonder, Celine Dion, Diana Ross and John Travolta.