How Prince Harry Leans on Kate Middleton

Kate Middleton and Prince Harry
Catherine, Duchess of Cambridge and Prince Harry stand on the balcony of Buckingham Palace following the diamond Jubilee Procession on June 5, 2012 in London, England. Anwar Hussein/

Kate and Harry are in cahoots!

You won't catch Prince Harry calling Kate Middleton his sister-in-law. "As he puts it, the 'in-law' part isn't necessary when he's describing her. He sees her more like a sister," a source tells Us Weekly. "He often turns to her for advice — they've always been close."

Which is why the cheeky royal, 27, isn't afraid to share his silly side with the duchess, 30. "She loves his sense of humor," says the source. "He makes her crack up all the time, even when it's not appropriate."

Prince William gets in on the fun too. After the June 4 Diamond Jubilee concert, the trio partied together until 1:30 a.m.!

And Kate and Harry (along with some of William's closest chums) planned a special party to mark the RAF officer's 30th birthday on June 21. One pal said Kate (who celebrated her own January milestone birthday with a quiet dinner at Kensington Palace) brought together a group of their friends for a night of fun. Another insider told Us, "Kate wants it to be really special — a get-together that William will never forget!"

Another memory from his 30th birthday William won't forget? Inheriting $15.5 million! The Duke of Cambridge inherited his share of his late mother Princess Diana's estate.

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