It’s been 12 years since Prince William and Kate Middleton said “I do” – marking a day that has since gone down in history as one of the royal family’s happiest moments. While initial thoughts turn to the royal bride and groom as they wed in Westminster Abbey, memories of a third person standing nearby, for whom the day held similar sentiment, linger. Standing proudly next to his brother, Prince Harry was visibly overcome with emotion, reportedly whispering to his older sibling, “Wait until you see her,” as Kate approached, and later paying tribute in his speech to “the perfect brother” and to the new in-law that he had long loved “like a sister”. Back then, the bond that he and William shared seemed bulletproof.

Twelve years later, that bond lies in tatters – and it’s hard to imagine the brothers ever standing side by side again. Though it’s difficult to determine when exactly their squabble turned to a full-blown feud, it’s fair to say that tension has been building for a while – exasperated, in no small part, by Harry and his wife Meghan Markle’s repeated attacks against the royal family, and specifically against William. Now, the once close brothers reportedly don’t speak, and the ocean that lies between them barely compares to their emotional gulf. But while neither side seems willing to repair the damage, we’re told that that Kate – who is said to have always loved Harry like her own brother – is stepping up. Sources say she’s decided “life is too short” and, with Harry turning 39, the time is nigh to make amends. And despite receiving some push back from her husband, she’s not taking no for an answer.

“For years, anytime that William and Harry would fight, it was Kate that would bring them back together,” says an insider close to the Duke and Duchess of Cambridge, who first met in 2001 and quickly became close to each other’s families. “She’s always been the natural peacemaker. But this time, even she hasn’t been able to make a dent in things. She’s not giving up though. She was so close to Harry for so many years and genuinely misses him, but she’s convinced they can be that way again. She’s obviously got to walk a very fine line – she can’t be seen as being disloyal to the Crown, so it’s all got to be done with the utmost care and diplomacy. But she’s determined to make it happen – for William’s sake. For all his anger, it’s obvious that this is causing him a huge amount of pain.”

Harry first lit the tinder box – alongside Meghan, 42 – when, having formally stepped down as working royals and moved to California, they sat down with Oprah Winfrey for a tell-all interview, in which they took aim at the alleged animosity, racism and lack of support they’d received from the royal family. Since then, they’ve only amplified their complaints, claiming that the Palace “fed Meghan to the wolves” and that William, 41, had left Harry “terrified” during their Sandringham Summit, held to discuss the details of Megxit, by “shouting and screaming” at him. In his memoir, Harry claimed his brother had “knocked [him] to the floor” after a heated exchange sparked by William’s supposed antipathy towards Meghan. Kate herself has barely escaped the accusations lobbied by the Sussexes, with both detailing her apparently icy behaviour. Also in his book, Harry claimed that Kate, 41, had looked “disgusted” after Meghan asked to borrow her lip gloss, and also told her new sister-in-law off for making a joke about her “baby brain.”

Still, despite the claims, Kate’s determined to build bridges between the two couples – who once, in the early days of Harry and Meghan’s relationship, were heralded as “the fab four”, promising the royalty a fresh and promising future. We’re told that Kate can no longer sit on the sidelines and watch her husband lose the person who used to mean so much. And now, a year since Queen Elizabeth’s death, she’s determined to do her late Majesty proud by repairing family wounds.

“Even in happy moments, it’s clear to Kate that there’s a huge weight on William’s shoulders,” says our source. “That’s why she’s ploughing ahead with her plan to bring peace to the family again – it’s what’s best for everyone. She has no doubt it’s what the Queen would want; if she were still alive, she’d never let this war between Harry and William go on. But with her gone, Kate really is the only hope of this ever getting sorted. Thankfully, she’s not the type to give up.”

Kate’s initial plan, we’re told, was to use Harry’s birthday last week as an opportunity to get in touch. The way she sees it, it’s not just the two brothers who are suffering from the fallout – her and William’s three children, Prince George, 10, Princess Charlotte, eight, and Prince Louis, five, plus Harry and Meghan’s two children, Archie, four, and Lilibet, two (who has never met her aunt and uncle) are also collateral damage. But she won’t let them lose out in the long term.

“She’s been telling William that this standoff really does nobody any good, and that Harry’s birthday is the perfect hook to break the ice,” we’re told. “The children will be sending Uncle Harry a card and presents, and Kate will ensure they call him on the day itself. There’s also talk of a private drinks bash being arranged next time Harry’s in London. If William wants to keep huffing and puffing, he can – but it won’t do his marriage any good.”

Among her close circle, Kate is described as “tenacious”, so perhaps she really will be the one to finally unite a broken brotherhood. Perhaps then, her memories of approaching her husband-to-be at the altar, as his loyal brother stood beside him, will no longer seem so far-fetched.