Kate Middleton’s younger sister is expecting her third child.

On Sunday, multiple sources confirmed to People magazine that Pippa Middleton and her husband, James Matthews, were expecting. The couple is parents to son Arthur, 3, and daughter Grace, 1.

A spokesperson for the British socialite, author and columnist didn’t immediately respond to Fox News Digital’s request for comment.

The 38-year-old was glowing during her appearance at the star-studded "Party at the Palace" concert on Saturday night outside of Buckingham Palace. For the occasion, Pippa wore a vibrant green dress that proudly flaunted her growing baby bump.

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Pippa Middleton third child

Pippa Middleton during the "Platinum Party at the Palace" in front of Buckingham Palace on June 4, 2022, in London. (Chris Jackson - WPA Pool/Getty Images)

Pippa’s parents Carole and Michael Middleton were also in attendance, as well as her brother James Middleton and his wife Alizée Thévenet. The Duchess of Cambridge was nearby at the Royal Box with her family.

The baby will be the sixth grandchild of Carole and Michael. Middleton, 40, and her husband Prince William share three children: Prince George, 8, Princess Charlotte, 7, and Prince Louis, 4.

Pippa and her beau, a financier, dated in 2012 and said "I do" in 2017. George and Charlotte were part of the bridal party.

Pippa rose to fame in 2011 when millions around the world watched her serve as Middleton’s maid of honor at the royal wedding. She wore a figure-hugging gown for the occasion that quickly went viral. That sparked completely unfounded rumors that she was dating Prince Harry.

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Pippa Middleton royal wedding

Catherine, Duchess of Cambridge and her father Michael Middleton arrive with Pippa Middleton for her wedding with Prince William at Westminster Abbey on April 29, 2011, in London. (Samir Hussein/WireImage)

She is notoriously private and rarely gives interviews. Pippa has written a book about entertaining and columns for Vanity Fair while also working at her parents’ lucrative "Party Pieces" business.

Back in 2016, sources told People that Pippa had "a deep desire" to disappear from the spotlight and live a quiet life while raising her children.

The Platinum Jubilee marked Queen Elizabeth’s 70 years on the throne. The four-day weekend was filled with festivities to honor the reigning monarch, 96. Elizabeth skipped several events due to ongoing mobility issues.

Pippa Middleton parents

Pippa Middleton with her parents Carole and Michael Middleton. (Samir Hussein/WireImage)

Ingrid Seward, the editor of Majesty Magazine, previously said the Middletons should be proud of their daughters. One married a future king and the other tied the knot with "an extremely wealthy, charming and handsome man."

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"You can’t do better than that," she said.

The Associated Press contributed to this report.