The Duchess of Edinburgh, 59, and the Princess of Wales, 42, are not only related through marriage but also closely bonded by their experiences of being "commoners" entering the royal family.
Sophie exchanged vows with Prince Edward in 1999 following a six-year relationship, with the royal admitting at their engagement photocall: "It's impossible for anyone else to understand why this has taken me so long [to propose], but I don't think it would have been right before and I don't think Sophie would have said yes before."
Kate similarly had a long-term relationship over eight years with Prince William before the couple walked down the aisle in 2011, earning her the nickname "Waity Katie".
William also confessed during his engagement interview that they had been discussing marriage for "a while" before he proposed in Kenya while on holiday with friends.
"We've talked about today for a while, we've talked about this happening so Kate wasn't in the dark at all when we were planning it for at least a year if not longer, it was just finding the right time and that's what most people say about couples, it's just about timing," he said.
Despite seventeen years separating the pair, this shared experience is said to have helped cement the bond between Kate and her aunt-in-law Sophie.
"Sophie was there for Kate in an unofficial capacity because she had the same experience of being a commoner marrying into the family," royal author Ingrid Seward told HELLO!.
"They both had really long romances with their husbands-to-be. By the time they got married, they had more knowledge than the royal brides before them about what they were getting into."
Meanwhile, royal author Robert Hardman added they had a similar "adjustment" period following their weddings.
"They are both people who weren't in the royal orbit before and had to adjust to that. The Duchess is not that far ahead in her motherhood journey - her son is still at school. And the two families now live close to one another," he said, referring to Kate's children Prince George, 11, Princess Charlotte, nine, and six-year-old Prince Louis, and Sophie's daughter Lady Louise, 21, and son James, Earl of Wessex, 16.
Ingrid, author of My Mother & I, the Inside Story of the King and Our Late Queen, described Sophie as "the older sister Kate never had" and added their approach to motherhood is similar.
"They are both protective of their children. Sophie has had a young family, so she knows what it's like trying to protect them while being in the royal world," she said.
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