[09:38] Pippa Middleton, sister of Catherine, Duchess of Cambridge, poses for photographers to promote her first book
Pippa Middleton, sister of Catherine, Duchess of Cambridge, poses for photographers to promote her first book "Celebrate", on the subject of party planning, in London October 25, 2012. Reuters/Neil Hall

Pippa Middleton, 31, showed off her washboard abs in a red bikini during a family holiday at Caribbean island, St Barts. Kate Middleton’s younger siblings, Pippa and her brother James Middleton, were spotted enjoying their vacation along with their mother, Carole Middleton, and a friend.

The brother-sister duo, their mother and friend were seen sailing on a pontoon with “Eden Rock – St Barths” scribbled on it. Pippa was wearing a red ViX bikini, showing off her toned abs and svelte frame.

The solid red Bia tube bikini features 24 karat gold-plated adjustable sliding tube detail, straps that tie behind the neck and back and comes in a lycra blended fabric. Meanwhile, the bottom features a similar 24 karat gold-plated adjustable sliding tube and ViX logo detail on back.

The bikini is a favourite of Kim Kardashian, Kelly Brooks, Halle Berry, Megan Fox and a host of other celebrities. The range of bikinis is known for its colour, cut and the metallic hardware trims detailing. The USP of the range is comfort, and it suits all body types.

Meanwhile, James was sporting a pair of bathing shorts, sunglasses and his trademark beard. Mum Carole opted for a white, blue and purple patterned one-piece, as she soaked up some sunshine with her children.

The Middleton family is spending some time at a St Jean Bay resort, Eden Rock. The luxury hotel resort is owned by the family of Pippa’s former boyfriend and investment banker James Matthews, Hello reported.

Pippa recently stepped in for sister, Duchess of Cambridge, and attended a society wedding of her brother-in-law Prince William’s friend Bear Maclean to socialite Daisy Dickson in Devon on Aug 15. At the wedding ceremony, she made a style statement in her floral pattern bespoke dress by Tabitha Webb, one of her favourite designers.

The brunette beauty also collaborated with the British designer to design a dress for the British Heart Foundation, of which she is a brand ambassador.