Princess Kate and the Duchess of Edinburgh were given "more knowledge" before making a key move, a royal commentator has claimed.
Kate, 42, and Sophie, 59, both married into the Royal Family from middle-class backgrounds and have young children, contributing to their close relationship.
The Princess of Wales and the Duchess of Edinburgh stood beside each other during Remembrance Sunday.
Following the service at the Cenotaph, Sophie placed a supportive hand on Kate's back as the pair walked back inside.
Princess Kate and Duchess of Edinburgh given 'more knowledge' before making key move
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Ingrid Seward, a royal author, explained why the two royal women are so close.
She said: "They have a huge amount in common having probably formed a bond very much at the beginning.
"Sophie was there for Kate in an unofficial capacity because she had the same experience of being a commoner marrying into the family.
"They both had really long romances with their husbands-to-be and Sophie lived at the Palace.
The Duchess of Edinburgh placing a supportive hand on Princess Kate's backPA"By the time they got married, they had more knowledge than the royal brides before them about what they were getting into."
The commentator continued to tell Hello!: "They have a sisterly bond and Sophie is like the older sister Kate never had.
"They are both incredibly kind girls and have a caring side to them. 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. Sophie also has experience of a life-threatening illness - she nearly died when her daughter Louise was born.
Kate and William have a very close bond with Sophie and EdwardPALATEST ROYAL NEWS:
The Duchess of Edinburgh and Princess Kate have a close relationshipPA"She has been there in the background, as part of a private support network."
Speaking Sophie placing a supportive hand on Kate's back, Seward added: "The royals have always shown a human side but there is more warmth now.
"That gesture of Sophie putting her arm around Kate is something you wouldn't have seen a few years ago."
Royal commentator Robert Jobson added: "That moment said a lot. The Royal Family are all looking after Kate and there is a protective bubble around her."
