Splash News
August 24, 2015 10:45 PM

Just one month after completing a scorching marathon in Africa, Pippa Middleton is cooling off on a tropical getaway as she prepares for “the mother of all swim-runs.”

Princess Kate‘s younger sister, 31, has been enjoying some family time in the Caribbean – with a rumored ex-boyfriend – before she embarks on her next grueling sports challenge.

Showing off her toned physique, the royal sibling was spotted being playfully thrown into the sea by her brother James Middleton and friend James Matthews at the luxurious resort Eden Rock on the French Caribbean island of St. Barts, while mom Carole Middleton looked on.

It’s clear the siblings inherited mum’s amazing genes as the royal grandmother stunned in a printed one-piece, while Pippa rocked a stylish red ViX bikini.

Pippa and Matthews, an investment banker whose parents own the five-star hotel, were spotted out for dinner in London in May and were rumored to be dating back in 2012, before she met her current boyfriend, Nico Jackson.

Having taken on the Race for America challenge last year, the pals are about to embark on another huge physical challenge – the legendary Otillo Swim-Run World Championship in Sweden.

The event, which takes place on September 7, will see teams of two race together from Sandhamn to Uto in the Stockholm archipelago. Racing across all 26 islands covering almost 47 miles, of which six miles are open-water, the dawn-to-dusk endurance race is ranked as one of the toughest in the world.

“It is a massive challenge,” Micheal Lemmel, co-founder of the Swedish event, tells PEOPLE, adding: “You need to be very fit and be mentally strong – she has trained very hard for it.”

Also running the course are Pippa’s vacation buddies: her brother, Matthews and Swedish pal Jons Bartholdsons.

Pippa has impressed the organizers with her dedication.

“I was pleased that she was willing to take on such a big challenge. She is very brave to do so,” says Lemmel, who has spoken to Pippa several times since she and team partner Bartholdsons signed up.

The four friends are taking part to raise money for the Michael Matthews Foundation, set up in honor of Matthew’s brother who tragically died while descending from the summit of Mount Everest in 1999. So far, Matthews has raised over $18,000.

As for how the super-fit Pippa might fare, Lemmel, who himself has taken part in over 70 of the world’s hardest adventure races, has high hopes.

“I’m sure she will be okay. Women only apply if they know that they will manage,” he says, noting that only 35 percent of the competitors are female.

While her training is “going well,” according to the event organizer, Pippa may well have to postpone any planned birthday celebrations on September 6 – the day before the big race. But just don’t expect any balloons at the finish line.

“She just wants to be treated as an equal,” says Lemmel.

And while she’s busy traveling for the race, she’s keeping her royal relatives close.

The doting aunt accidentally revealed a candid picture of Prince George and big sister Kate while leaving London on July 24 when a fellow traveler passed her on the escalator at Heathrow Airport.

“She pulled out her phone and her lock screen had a picture of George and Kate, smiling together looking just like a regular family – it was so cute!” Grace Stroup, 16, told PEOPLE.

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