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Pippa Middleton, beau to vacation on Mustique

Pippa Middleton and Alex Loudon attend the quarterfinal round match of the Wimbledon Lawn Tennis Championships, June 29, 2011. Getty

It looks as if Pippa Middleton isn't quite back on the market.

The younger sister of Catherine, Duchess of Cambridge, is heading to the Caribbean with on-again, off-again beau Alex Loudon.

Pictures: Pippa Middleton
Special Section: The Royal Wedding

According to the British newspaper The Sun, Middleton, 27, is taking Loudon, 30, on a private vacation later this month to the island of Mustique. A source told the newspaper the two are trying to work out some relationship kinks that have arisen since Middleton made headlines as the maid of honor at her sister's wedding in April.

The private 2.2-square mile island in the Grenadines is a favorite of celebrities such as Mick Jagger, Kate Moss, Hugh Grant, David Bowie and Tommy Hilfiger.

It was a favorite resort of the Queen's sister, the late Princess Margaret, and Middleton's sister Kate and Prince William spent a vacation there in 2007. Queen Elizabeth II and Prince Philip have visited the island three times.