So what on earth can be in the Queen's mind, asking a woman who, more than once, has been accorded the status of Public Enemy No 1? Why risk incurring the wrath of your husband, let alone a significant swathe of your subjects? One likely explanation is that we shall shortly be hearing another nuptial announcement, this time concerning Princess Beatrice, 25, and her long-term boyfriend Dave Clark. If so, we can expect a royal wedding as befits the sixth in line to the throne, probably at St George's Chapel, Windsor.
The Duchess's people are denying such an eventuality - "not even close", they say - but it's hard to see otherwise why this extraordinary rapprochement should take place, and publicly.
Beatrice celebrated her birthday last week with a party at the Yorks' country residence, Royal Lodge - where Fergie lives - before heading for Balmoral with her parents. With the birth of Prince George of Cambridge pushing her further down the pecking order, and with the Prince of Wales's avowed intention of slimming down the royal machine in the coming years, it looks like Beatrice is largely surplus to royal requirements. A good time, then, to concentrate on marriage: she has been with businessman Clark, who works for Sir Richard Branson, for the past six years.
This conjures the prospect of the iceberg up ahead: the wedding. The Duchess of York, as Beatrice's mother, should be accorded appropriate status on the day. But she has blotted her copybook so often, how, her critics might ask, could she seriously think of playing a significant part in the ceremony?
So here is the Queen's dilemma: should she embrace Fergie? Or leave her out in the cold and risk her kicking down the chapel doors in an attempt to take her rightful place in the congregation?
We know the answer to that - it lies in the Queen's pragmatism. Whatever her private feelings, she's always shelved them in preference to ensuring that her family presents a united front to the outside world: to do otherwise would be to risk the charge that she ruled over a divided house, with all the historical baggage that goes with such an accusation.
Charteris once said: "She is enough of a realist to know there's nothing for it but to sit it out. She believes the monarchy is strong enough to withstand change and analysis."
And so it is with Fergie. The time has come, for whatever reason, to welcome her back into the fold, and so both change and analysis arrive at the doors of Royal Lodge. The Queen's startling embrace of her erstwhile daughter-in-law is of some significance, and will lead inevitably to further questions about quite what goes on between Sarah and Andrew.
Since their divorce in 1996, both have had relationships, however shallow, with others. That they should go on relishing each other's company seems extraordinary. Certainly, they are joined in their love for their two daughters, and Andrew follows very much the Queen's line that: "I may criticise my family; others may not."
One who is close to the House of York explains the Balmoral phenomenon thus: "They live together as a family unit and that's probably what this is more about than anything else." There could be no more to this detente than forgive and forget - after all, it's been three years since the Duchess was caught in a tabloid sting, offering access to her husband for 500,000. She has since kept her lip admirably buttoned.
There's also the matter of public opinion. The Queen may forgive, but can others? Whatever the reason, it's clear that Fergie has been given another chance.
