Britain's Prince Harry "often" asks his sister-in-law Duchess Catherine for advice.
The 27-year-old royal sees the 30-year-old brunette - who tied the knot his brother Prince William in April last year - as a proper sibling rather than someone who has married into his family.
A source told Us Weekly: "As he puts it, the 'in-law' part isn't necessary when he's describing her. He sees her more like a sister.
"He often turns to her for advice - they've always been close."
As well as being on the receiving end of advice from Catherine - who was known as Kate Middleton prior to her nuptials last year - Harry returns the favour by making her and William laugh "all the time" with his cheeky sense of humour.
The source added: "She loves his sense of humor. He makes her crack up all the time, even when it's not appropriate."
Harry and Catherine recently helped Prince William celebrate his 30th birthday with a small gathering at the Duke and Duchess of Cambridge's rented cottage in Anglesey, North Wales with some of William's lifelong pals including Thomas van Straubenzee, Harry Aubrey-Fletcher and David Jardine Paterson.