Capturing the imagination of the public at home and around the world, the two young royal couples – William and Kate, Harry and Meghan -- are the new “fab four” who are catapulting the monarchy into the 21st century.
“The kind of royal Beatles -- Harry, William, Meghan, and Catherine. They are seen as authentic stars, the people who will drive the monarchy forwards,” Andrew Morton, a royal biographer, told “20/20.”
Together, they are seen as the full package.
“They've got the glamour, they've got the relatability,” Emily Nash, royal correspondent at “HELLO!” magazine, told “20/20.”
Diana and Sarah, who got along famously, stole the spotlight, breaking royal protocols even before Sarah’s wedding.
“Fergie arrives. This light. This fun. So Diana starts acting up,” said Webber. “They both got dressed up as policewomen and crashed Prince Andrew's bachelor party. You're not supposed to do that.”
Eventually, the duo adjusted to rules established by the palace.
"The most difficult thing for Princess Diana and Fergie to deal with was just the fact that this was a family that is not like any other family. They have all these walls, all these protocols,” Victoria Murphy,Daily Mirror's royal correspondent and ABC News contributor, told “20/20.” “They were sort of thrown into the deep end already and they very much supported each other in that.”
But the 20-something royals couldn’t stop trying to make their duties fun.
“Diana and Fergie being mischievous was when they're at Royal Ascot, which is very proper in the royal enclosure where you're wearing your hats and everything. And they're poking friends with their umbrellas. Not the done thing,” said Webber of their 1987 antics caught on camera.
Partners in crime, the two shared a deep friendship.
“We were always low and steadfast to our friendship. We always rolled with laughter,” Sarah Ferguson said in an interview to “Primetime” after Diana’s death. “We've laughed and smiled and cried and every day sought each other's advice.”
More than three decades later, a new pair of royal sisters-in-law is becoming iconic in their own right.
“While William and Harry have been the stars of the show when it came to the younger generation, for a long time, now it's their women who are really the focus of all our attention,” Nash said.
Kate and Markle’s closeness is a result of the strong bond shared by brothers William and Harry who have been inseparable since childhood, royal experts said.
“William is Harry's closest friend. He's the person who he has shared most with throughout his life,” said Nash. “So you can imagine that the four of them have spent a great deal of time together.”
“This incarnation of the fab four, a lot more functional. You've got four people in their mid-30s who absolutely know and laser focused on that they want to achieve,” said Webber.
Their personalities have also been helpful in strengthening the family bond.
Markle and Harry’s first joint public appearance on Christmas, a month after the royal engagement, made history -- Markle became the first royal fiancée to be invited to Sandringham House, the queen’s country home.
“It was rather unprecedented. Meghan, who is only just engaged to Prince Harry, was allowed to attend church, [she gave a] curtsy to the queen in front of the world. The first time we ever saw her do that,” said Webber.
It is believed that Kate has been personally helping the future bride navigate royal protocol.
“She's the only person who can really understand what it is like to marry a prince, have a huge royal wedding, and then start being a working member of this royal family,” said Murphy.
Fashion is the other big arena in which Kate, a huge icon and influencer in British fashion, could be of immense help to the American actress, royal commentators said.
“When Kate wears something, it's worth $1.4 billion a year to British fashion -- The Kate Effect. And we expect ‘The Meghan Effect’ to sort of be in that range,” said Webber.
The fab four, however, are not immune to the universal challenges of marriage, such as dealing with family members. Most recently, Markle’s father, Thomas Markle Sr., became the subject of scandal when a report in a British tabloid alleged that he had staged photographs with a paparazzi photographer. Thomas Markle then reportedly said he would no longer attend the wedding, only to change his mind a day later, according to TMZ. Whether he will attend is still unclear, as new reports of health concerns have surfaced.
At The Royal Foundation in February, Harry spoke of “healthy disagreements” among family.
“Working as family does have its challenges, of course it does,” he said. “The fact that everybody here is laughing means everybody knows exactly what it’s like. But you know we’re stuck together for the rest of our lives.”
“Togetherness at its finest,” added Meghan Markle.
The four royals are also neighbors, living on the same grounds where the princes were raised.
Apartment 1A, the four-story mansion that is home to William, Kate and their three children, is a stone’s throw away from Nottingham Cottage, the two-bedroom home of Harry and Markle that was, incidentally, the first home of the older prince when he first got married.
“With Prince George and Princess Charlotte, and now, Prince Louis, Auntie Meghan, as she will be soon, is going to play a key part in their lives,” said Nash.
The new couple may even start a family soon, royal commentators speculated.
“Children is 100 percent on the agenda. Harry's made no secret of the fact that he wants to start a family,” said Murphy.
Royal experts believe the couple will likely raise their children with the same values passed on by Harry's late mother, Diana.
“I want them to have an understanding of people's emotions, people's insecurities, people's distress, people's hopes and dreams. And they have a knowledge. They may never use it. But if the seed is there, then I hope it will grow because knowledge is power,” Diana said in her famous 1995 BBC News Panorama Interview.