Good-natured scene stealing is nothing new for Pippa Middleton.

Although Duchess Kate and her suddenly-famous younger sister and Maid of Honor, 27, are "best friends" according to one family friend, that doesn't mean they're unlike most siblings. "There is a healthy competition between the two," royal watcher Katie Nicholl says in the new Us Weekly.

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"Certainly when it comes to looks, like who is the slimmest or better looking, there is a little friendly rivalry."

Kate, 29, and Pippa squabble over "weird things," says a family source. "You'd see them fight over who had the worst headache or period pain!"

And it's no wonder that Pippa has grabbed headlines for her sexy, elegant style, her chemistry with Prince Harry during the royal wedding -- and startling pics of her partying in a purple bra and a white bikini.

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"She was the sister who sparkled," Nicholl, author of The Making of a Royal Romance, says. "She is an extrovert, compared to Kate."

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Indeed, as fellow students at posh boarding school Marlborough College, "none of the guys noticed Kate," a source says. When Kate began attending St. Andrews in Scotland (where she met Prince William), Edinburgh student Pippa frequently visited to mingle with her sister's noble new pals.

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"She dated a couple of the St. Andrew's lads and tried to get in with a lot of William's friends," the source says. "She wasn't sleeping around, but she flirts with the most well-connected men."

For much more on all the Middletons -- Pippa's wedding plans (and Harry's!), Kate's dreams of a baby, and whether Kate "chased" her Prince -- pick up the new Us Weekly, on stands today.

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