pippa middleton at kate middleton wedding

Pippa Middleton’s fit physique and elegant beauty made headlines at her sister’s 2011 wedding. (Getty Images)

Pippa Middleton, the sister of the Duchess of Cambridge Kate Middleton, flashed a big smile and a huge diamond engagement ring (see photo below) after announcing she will marry hedge fund executive James Mathews.

Not surprisingly, Kate Middleton and husband Prince William are thrilled about Pippa’s upcoming wedding.

“The Duke and Duchess of Cambridge are absolutely delighted,” said a Kensington Palace rep.

Mathews proposed the old-fashioned way: He got down on one knee during a weekend trip in northwest England (see video).

Pippa, 32, dated Mathews, 40, in 2012 but broke up several months later. They started dating again in 2015. Mathews runs Eden Rock Capital Management in London.

Pippa stole the show at her sister Kate’s 2011 wedding, when she wowed the worldwide TV audience with her incredible figure.

Middleton, a lifelong fitness fanatic, stays fit by following a low-carb diet and working out 3 to 5 days a week, doing Pilates and running, Celebrity Health Fitness reported.

While many celebrities embrace the Paleo, ketogenic and Atkins diets for weight loss, Middleton prefers her own modified version of a low-carb diet that limits bread and pasta.

“I do believe in making healthy choices: keeping refined carbs such as white bread and pasta to a minimum, and sticking to sensible portion sizes,” said Pippa.

Low-carb diets such as the ketogenic and Atkins diets promote weight loss by inducing the body to burn body fat for fuel, said obesity expert Dr. Eric Westman, co-author of Keto Clarity.

Physician and weight loss expert Dr. Mark Hyman agrees.

By reducing carbs and eating more healthy fats, you can accelerate weight loss and enjoy consistent energy throughout the day, said Dr. Hyman, author of Eat Fat, Get Thin.

Pippa said regular exercise is a part of her healthy lifestyle, saying working out makes her look and feel her best.

“I know that if I fit in at least three, ideally five, sessions of exercise a week, be it a 30 minute run, an hour’s walk or a good game of tennis, it boosts my mood and energy and helps me sleep and digest better,” said Middleton, who credited Pilates workouts for her hot bikini body.

Pilates is an excellent low-impact way to lose weight and tone your entire body, said fitness expert Cassey Ho, author of Hot Body Year Round.

Middleton said she keeps her glutes toned by focusing on four key exercises: ladders, plie squats, wall pushes and the classic plank.

“Do the routine three times a week and you’ll really notice a difference,” said Pippa.

Related: Jessica Alba’s bikini body diet and exercise secrets