Their relationship has long been strained, but the latest revelations from the Epstein files could further deteriorate the Prince of Wales's relationship with his cousins, Princesses Beatrice and Eugenie.
The most recent tranche of documents has raised serious questions about how much Andrew Mountbatten-Windsor and Sarah Ferguson's daughters knew of their parents' corrupt dealings with the paedophile financier.
One email sent by Epstein in 2015 - after he was convicted for soliciting sex from girls as young as 14 - boasted to a friend that Beatrice 'liked' him.
It also emerged that Beatrice helped advise her mother on how to placate Epstein after Fergie called him a paedophile while apologising for accepting a £15,000 bailout from the convicted sex offender in a 2011 interview.
She also played a key role in facilitating her father's 2019 Newsnight interview, and Beatrice and Eugenie must now face the possibility that the drip-feed of Epstein revelations could see them ousted from royal life as well.
The future king, 43, now must evaluate his relationship with Beatrice and Eugenie, and assess whether they have a future within The Firm - a decision no doubt fuelled by the pre-existing tension in his relationship with the York sisters.
Speaking to the Daily Mail, royal commentator Richard Fitzwilliams said: 'It may fall to William to make a decision on the future of Beatrice and Eugenie. Neither are reportedly particularly close to him or to Catherine.
'The fact that Beatrice, when 20 and Eugenie, aged 19, visited Epstein in Florida just after his release from prison with their rapacious mother, is alarming. They must have known who they were visiting and how inappropriate this was. He was under house arrest.
'One of the most bizarre aspects of the Files is the way Sarah Ferguson seems to regard Epstein as part of her extended family. In 2010, she even told Epstein of Eugenie's "s*******' weekend".
'There is obvious sympathy for the predicament of the sisters; mention in the files does not denote wrongdoing.
'The King reportedly regards their work-life balance, where they both do some charitable work alongside their careers and families, but are not working royals, as admirable.
'William, it is reported, may well take a hard line when he becomes king. It depends on how affected the royal family is by what happens subsequently.
'There is currently the strong possibility of the cases which the police are assessing going forward and Andrew Mountbatten-Windsor being arrested and very possibly put on trial.
'If this happens, it may be impossible for the Royal Family to move forward with any members of the York family attached to it.
'William and Catherine do not appear to have close ties with the sisters and would probably not miss them.'
Rumours of tension between the pair go back as far as 2011, when the future king failed to invite Beatrice's long-term boyfriend, Dave Clarke, to his wedding.
It was a surprising twist, given that William introduced the former couple to one another at a party hosted by Sean Brosnan, the son of film star Pierce Brosnan.
Fitzwilliams said: 'It was William who introduced Beatrice to her former boyfriend Dave Clark, whom he had met through mutual friends in Scotland. They dated for a decade.'
In spite of having made the initial introduction, William had cooled his friendship with Dave, seeing him as 'indiscreet', to the extent that he didn't ask him to his wedding in 2011 - a decision that hurt Beatrice very much.
'Reports indicate that William fell out with him, regarding him as unreliable,' Fitzwilliams said, adding: 'Proof of this is his absence from William's wedding in 2011, which was obviously one of the weddings of the century.'
A source told the Daily Mail in 2016: 'William isn't a huge fan of Dave. It's fair to say that if Beatrice had asked the Queen for permission to marry Dave, and she in turn asked William privately for his view, she may not have been given a glowing report.'
Fitzwilliams continued: 'Although the Waleses may not be close to the sisters, we know that Eugenie is notably close to Harry.
'At one time it was thought that she had a role to play in helping reconciliation between the Royal Family and the Sussexes. That is now highly unlikely.'
Over the years, Eugenie has been Harry's close friend, confidante and even matchmaker.
Despite being born five years apart, Harry and Eugenie often spent time together as children thanks to the friendship between their mothers, Princess Diana and Sarah Ferguson, who would pack the children up for joint family holidays.
Eugenie was the one who introduced her cousin Harry to his ex-girlfriend, Cressida Bonas, and they dated between 2012 and 2014. Eugenie and the actress have remained close after the breakup.
'Out of all the Queen's grandchildren, Harry and Eugenie have one of the most natural connections,' Omid Scobie and Carolyn Durand wrote in the Sussex biography Finding Freedom. The book also describes them as the 'closest of friends'.
Harry has always confided in Eugenie, and she was reportedly among the first to know about his relationship with Meghan, whom she already knew through their mutual friend Misha Nonoo.
'Not only did he trust her implicitly, but friends said that she gives great advice and has always been "beyond wise" for her years,' Scobie and Durant wrote.
Eugenie and Jack Brooksbank, who married in October 2018, just six months after Harry and Meghan, went on double dates with the couple in Toronto and London.
The Princess and her then-boyfriend were at a party in Soho House in Toronto in 2016 when Harry and the Suits star were first seen together. Eugenie was instrumental in keeping their relationship secret.
In the explosive tell-all Oprah Winfrey interview, in which Harry and Meghan criticised the Royal Family, Eugenie was singled out for praise.
Meghan said: 'Eugenie and I had known each other before I had known Harry, so that was comfortable [...] We're friends with them as a couple.'
William, on the other hand, has continuously demonstrated his closeness to his cousin Zara Tindall, as opposed to Beatrice and Eugenie.
For instance, in an episode of Mike's podcast, The Good, The Bad and The Rugby, William admitted: 'The only time I've ever cried when watching sport was when Zara won, I think it was the European championship.
Zara, born just 13 months apart from William, was reportedly a great comfort to the future king after his mother Diana's death in 1997.
While she may not be a working royal and has experienced less media attention than William and Harry, the royal cousins have become protective of each other over time.
In a Channel 5 documentary, 'Zara & Anne: Like Mother Like Daughter', Victoria Arbiter said: 'For William in particular, trust is everything, so when you've got a cousin that understands what it is to be a member of the Royal Family, but they don't have the same pressures associated with it, they can be a tower of strength and support.
'It's Zara's loyalty and total dedication to her family that has made her such an asset to the cousins who have had to endure the negative side of royal life.'
Their bond is no doubt strengthened by Kate's closeness to Zara, and the pair have been captured at various events over the years, giggling or sharing a deep conversation.
Any concerns that Kate and William have about the York sisters will only be aggravated by their parents' continued association with Epstein after he was jailed in 2008 for soliciting underage sex from girls as young as 14.
The recent release of files connected to Epstein - who died by suicide while awaiting trial for sex trafficking charges in 2019 - contains further damning evidence of their friendship with the billionaire sex offender yet.
Astonishing emails unearthed by The Mail on Sunday revealed Fergie was so desperate to cosy up to the convicted paedophile that 'she was the first to celebrate' his release from jail 'with her two daughters in tow'.
Emails reportedly show the group met at Epstein's Palm Beach mansion, where the paedophile served vegetable lasagne prepared by a Parisian chef.
The Daily Mail has also found an email from Epstein in 2015 where he told a friend not to worry about meeting Beatrice at an event in Mexico because she 'liked' him. The paedophile famously attended her lavish Victorian-themed 18th birthday masked ball at Royal Lodge with Ghislaine Maxwell and Harvey Weinstein in 2006.
Princess Beatrice also supported her father's car-crash Newsnight interview about Epstein and Virginia Giuffre which is 'absolutely cataclysmic' for any hopes she has of having official royal duties when William becomes King, another expert said.
A BBC producer revealed that Princess Beatrice had attended a negotiation meeting with her father ahead of the interview in November 2019.
As well as attending a pre-production meeting with producer Sam McAlister, with a notebook and pen, Beatrice famously became Andrew's 'alibi' during the interview when he claimed he was collecting her from a party at Pizza Express on the night he supposedly had sex with Ms Giuffre.
In an email leaked to The Mail on Sunday, Fergie 'humbly apologised' and told the sex trafficker she knew he was feeling 'hellaciously let down by me' after the interview.
She also discussed how to restore his reputation with Beatrice, two years after they travelled to Epstein's Palm Beach mansion with Eugenie.
While they previously seemed to be bystanders caught up in their parents' relationship with Epstein, it now seems that Beatrice - according to Fergie - at least took a more active role.
Earlier this month, sources close to the sisters said they were 'appalled' and 'embarrassed' by a photo of Andrew crouching over an unidentified woman lying on the floor in the newly released files.
They are believed to be 'aghast' at their mother's sycophantic emails, calling Epstein a 'legend' and proposing marriage to him, and Eugenie must surely have been left mortified after her mother told the sex offender she was off on a 's***ging weekend'.
A source close to Eugenie and Beatrice told the Daily Mail: 'They are aghast at what they have read. They are mortified by the emails their mother has sent to Epstein. It is so embarrassing for them.'
They added: 'We don't believe the girls [Beatrice and Eugenie] were told much about what has just emerged [in the latest Epstein files release], and they will simply be aghast at just how close their parents were to this appalling man.'
