A daughter's wedding is always a joyful occasion that brings a family together, but Princess Eugenie's could do a lot more.
Her marriage to vintner Jack Brooksbank (32) at St George's Chapel at Windsor Castle on October 12 seems set to be the day when her mother, Sarah, Duchess of York, is finally accepted back into the royal family. It could even signal that her remarriage to Prince Andrew has now become a possibility.
"Princess Eugenie's wedding will, if you like, be a final test," says a courtier. "A lot of water has flowed under the bridge since the Duke and Duchess of York divorced. Her Majesty did see the duchess' point of view and has always had a soft spot for her.
"It is the Duke of Edinburgh who has been the sticking point around her return to the fold. He blamed the duchess for everything and is not a man who forgives lightly, but even he has mellowed with the passing years. It will be interesting to see if he actually talks to her. If he does, all well and good."
All the senior royals, including the Queen (92) and Prince Philip (97), will certainly attend. A smattering of celebrities will go, among them George and Amal Clooney as Jack is an ambassador for George's Casamigos tequila.
The Queen did not publicly acknowledge Fergie (58) for years after the Yorks' divorce in 1996, but more recently has gradually taken her under her wing once more, inviting her to Balmoral with Andrew (58) and their daughters Beatrice (29) and Eugenie (28).
And for the last four years, Fergie has been invited to Royal Ascot, the Queen's favourite race meeting. This year, she even made it on to the Queen's viewing stand and was seen chatting to Her Majesty, after curtseying so low that her knee almost touched the ground.
The day out was also notable for the closeness between Andrew and Fergie. They laughed at each other's jokes, with the duchess grooming her ex-husband by picking bits of fluff off his morning coat.
Judi James, a body language expert, notes, "It's a quasi-maternal gesture of ownership and the way Andrew keeps his hands clasped behind his back, and watches her face as she performs it, suggests he's either used to this behaviour from her or at the very least enjoys it.
"They might have divorced more than 20 years ago, but the body language rituals performed by Andrew and his ex-wife are still almost identical to those they used during their engagement interviews."
The former couple's living arrangements are close as well, with the duchess still sharing homes with her ex. At his UK base, Royal Lodge in Windsor, her rooms are just down the corridor from his. And at the $25 million chalet they bought together in the Swiss ski resort of Verbiers in 2014, their rooms are said to be adjoining.
One insider says, "She used to stay in a different house, but she sold it and moved in after the Queen Mother died.
"It's a strange relationship; no-one really knows, even I don't quite know how it all works."
Fergie has described it by saying, "I think our story is one of the great loves – definitely one of the great love stories of the royal family."
She didn't want to divorce but "had to because of circumstances".
Most people will think that's a reference to photographs of the duchess having her toes sucked by her financial advisor John Bryan during a French holiday.
In fact, the split was caused by Andrew's naval career. In the first five years of their marriage, the couple saw each other for less than six weeks a year. Andrew admits he was at least half to blame by neglecting his wife – and he's been making up for it ever since.
Andrew – and the Queen – also appreciate that despite Fergie's financial ups and downs, she's been true to her promise that she would make her own way after the divorce in 1996. It had been expected that she'd ask for a big pay-out.
She told one interviewer, "When I met Her Majesty about it, she asked, 'What do you require, Sarah?' and I said, 'Your friendship', which I think amazed her because everyone said I would demand a big settlement.
"But I wanted to be able to say, 'Her Majesty is my friend'; not fight her nor have lawyers saying, 'Look, she is greedy.' I left my marriage knowing I'd have to work. I have."
Now financially stable, Fergie still works like a demon. The day after attending Harry and Meghan's wedding, she flew to Las Vegas to promote her series of children's books, Little Red, which is about a brave rag doll.
Her earnings will contribute to the cost of Eugenie's wedding. While the royals will pick up the majority of the expected more than $1.1 million tab and Jack's family will chip in, the gesture will not go unnoticed.
"Will they remarry?" says a family friend. "Nobody really knows. I doubt they will while the Duke of Edinburgh is still alive, but we'll see. If they do, it'll be very quiet."
And the most likely venue? St George's Chapel, where divorcées Charles and Camilla had their wedding blessed.