Duchess of Cambridge Kate Middleton's sister Pippa Middleton has been enjoying some relaxing time with her family on the French Caribbean island of St. Barts, and pictures from her vacation show her rocking in a stylish red bikini.

The 31-year-old sister of Duchess of Cambridge Kate Middleton was accompanied by her brother James and some friends, reports noted.

Pippa, who is known for sticking to her exercise routine religiously, will be taking part in the legendary Otillo Swim-Run World Championship in Sweden on 7 September.

The championship will see teams of two taking part in a dawn-to-dusk endurance race, which is ranked as one of the toughest in the world.

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

"I'm sure she will be okay. Women only apply if they know that they will manage," he added.

Pippa has put a lot of hard work into achieving her toned body, and she recently opened up about how important exercise was.

"Exercise is a massive part of my life and a very personal thing for me; it's so important for both the mind and body," said Pippa, according to Daily Mail.

She also credited playing tennis with helping her get in shape. "Tennis has to be one of the best long-term fitness hobbies," she said. "It's relatively low-impact, dynamic and works the whole body (arms, legs, bum and core). I also find it a great release after a stressful day."

A Now Magazine report even claimed that Pippa's toned body inspired Kate to get back in shape soon after her second delivery.

"Kate wants to get fitter and have a few muscles. She's very envious of her sister's body," a source told the media outlet in June.

"When the Middletons were holidaying in Mustique in February, Kate was in awe of her sister's figure after she saw her in her bikini. She made a pledge to herself that she'd start working out as soon as possible after Princess Charlotte was born."