The couple once avowed their open marriage -- he fathered a child with their former housekeeper and she openly stated her preference for women -- but their love and loyalty for one another is still apparent after 28 years.
"However absurd their relationship may appear, there's never any question that they are very, very bonded," writer and film professor David N. Meyer, who is writing an unauthorized biography on the Bee Gees, told ABCNews.com. "He has relied on her in a number of ways. She is no joke to him. Their love for one another is very tight."
"They are an example of a very modern family -- maybe a little too modern," Us Weekly's music editor Ian Drew told ABCNews.com. "They are both very artistic souls, very open-minded. They get each other and they get a kick out of each other."
Their lifestyle, though, has raised more than a few eyebrows -- most recently, in 2009, after Robin, 62, fathered a baby girl, Snow Robin Gibb, with their live-in housekeeper, Claire Yang, a woman nearly half his age.
Despite their unconventional marriage, Dwina Gibb, 59, an artist, writer and druid priestess, was said to have "hit the roof."
"When the truth came out, Dwina was furious. To say she hit the roof is an understatement. She felt betrayed," a friend was quoted telling the Sunday Mirror.
"It's very funny that she was upset," Meyer said. "Maybe it goes to the king splitting his estate."
Robin Gibb, whose estate is reportedly worth more than $140 million, and Dwina Gibb have one son, Robin-John Gibb, 29. Robin Gibb also has two older children -- a daughter, Melissa, 37, and a son, Spencer, 39, with his first wife, Molly Hullis.
His 12-year marriage to Hullis, a secretary in former Bee Gees' manager Robert Stigwood's office, almost immediately fell victim to the rise of the Bee Gees.
"Almost as soon as they got married, Robin moved to America and they almost never saw each other," said Meyer, author of "The Bee Gees: The Biography," due out this fall. "She refused to bring the kids to live in America."
She also won custody of the children and, for many years, Robin Gibb did not see them.
"It was akin to bereavement," he told the UK newspaper The Telegraph in 2008. "I felt as though I was on the verge of madness."
Eventually, he reunited with his children when they were 12 and 10. By then, he had married Dwina Gibb, his second wife, whom he met through her cousin in 1980, when she was running a beanbag factory in London while trying to make it as an artist.
"I showed him my drawings," she is quoted saying in a 2006 article in the U.K.'s Daily Mail. "He asked me to come house-hunting with him and we scampered in and out of houses together, getting to know each other. We had a lot in common. We are both interested in history, mythology, old churches and buildings. We even share the same birthday."
Raised in Northern Ireland, Dwina Gibb has a lifelong interest in Irish history and mythology, according to her website, and has published two volumes of poetry and two novels. She's a devotee of many religions, including a Hindu sect called the Daughters of Brahma, whose members are meant to be celibate, and the order of the druids, an ancient pagan practice, for which she was ordained a patroness in the 1990s.
The couple live in the Biscayne Bay mansion once owned by President John F. Kennedy and a 100-acre Oxford, England, estate, where tapestries and tarot-card tiles adorn the walls of their 12th century converted monastery and the Gibbs built a druid place of worship.
The estate was also where Robin Gibb's younger brother, pop star Andy Gibb, spent his last days before he was rushed to a hospital, where he died of a heart ailment. And it was where Robin Gibb carried on an affair with the housekeeper, until his wife learned about the pregnancy.
Yang and her daughter now live in a converted barn not far from the estate and, Meyer said, Robin Gibb provides financial support.
Unlike Arnold Schwarzenegger and Maria Shriver, who separated after news that he had fathered a son with their housekeeper became public, the Gibbs remain committed together -- in love. Since the birth of Snow Robin, they have also backed off from earlier statements they made about their infamous lifestyle.
In the 1990's, Robin famously told Howard Stern that he and his wife "like to cruise and we like to watch."
Amid talk about threesomes, he declared, "My wife is a lesbian and I love it."
But in a 2010 interview with the Daily Mail, the couple disavowed their previous statements.
"My earlier life was kind of wild, interesting and experimental, but you go past the experimental stage and start living," Dwina Gibb said.
"You get older and other things are more important," Robin Gibb said, adding that he had never actually seen his wife "indulge in a bisexual lifestyle."
"We actually have a very conservative relationship -- more so than the average couple. We don't drink or smoke. We're not partying all the time," said Robin Gibb, who has been sober and a vegan for decades. "Look, we're not stuffed shirts. We have a free relationship. We give each other time and space to pursue activities -- that doesn't mean other people."
So how did he explain his daughter, Snow Robin? He didn't.
"This isn't about her [Dwina's] relationship with her [Claire], or mine," he said. "It's about the relationship we have with each other, and all I can say about that is it's rock-solid and that's the most important thing to me. I've never felt anything different."