Sophie is thought to have a lot in common with the Queen, from shared hobbies to living near each other.
It is also thought Sophie has stepped in to support the monarch at events since Prince Philip retired in 2017.
However, body language expert Judi James explained their close relationship was not always obvious.
When Sophie married Prince Edward, there was speculation on how she would be received in the family.
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The expert told Express.co.uk: “When Sophie married Edward she was hailed as the new Diana.
“However, what followed looked like a masterclass in hunkering in as the soap-opera-free royal bride who managed to be a stabilising influence on her husband while keeping press and royal fan attention to a minimum.”
When Sophie joined the family, Judi explained she seemed to handle royal life well.
The mother-of-two kept a low-profile while also serving the monarch as a full-time royal.
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Her attitude towards royal life could have been a turning point in her relationship with the Queen.
When the royal ladies appear out together, they seem comfortable in each other’s company, Judi continued.
“The Queen’s body language with Sophie suggests her daughter-in-law is a bit of a confidante,” she said.
“Sophie’s relaxed, constantly calm and upbeat demeanour seems to allow the Queen to relax more in her company, too.
“In terms of her body language she tends to blend some of the signals of the kind of laid-back calm and fun that we get from Zara with the stoic, deft formality of the Queen herself.”
Sophie and the Queen live near each other, both being based in and around Windsor.
In the past, it has been reported Sophie and the Queen would regularly spend time together in their homes.