The younger sister of Kate Middleton is expecting her third child with her husband James Matthews, multiple sources confirm to PEOPLE. The couple is parents to son Arthur, 3, and daughter Grace, 1.

Pippa was glowing in a green dress that revealed her baby bump at the star-studded "Party at the Palace" concert on Saturday night outside of Buckingham Palace.

A representative for Pippa declined PEOPLE's request for comment.

She was joined by Matthews, her brother James Middleton and his wife Alizée Thévenet. Her parents Carole and Michael Middleton were also in attendance at the event, which celebrated Queen Elizabeth's 70 years on the throne. Pippa's sister Kate and her family were nearby in the royal box.

LONDON, ENGLAND - JUNE 04: Pippa Middleton (L) and James Middleton (R) during the Platinum Party at the Palace in front of Buckingham Palace on June 04, 2022 in London, England. The Platinum Jubilee of Elizabeth II is being celebrated from June 2 to June 5, 2022, in the UK and Commonwealth to mark the 70th anniversary of the accession of Queen Elizabeth II on 6 February 1952. (Photo by Chris Jackson - WPA Pool/Getty Images)

Chris Jackson - WPA Pool/Getty Pippa Middleton, James Matthews, James Middleton and Alizée Thévenet

Pippa, 38, and James, 46, welcomed their first child — a son named Arthur — in Oct. 2018. She gave birth to a baby girl named Grace in March 2021. She followed in the footsteps of her royal sister by giving birth in the Lindo Wing of St. Mary's Hospital — the same place where Kate and Prince William welcomed all three of their children.

The baby will be the sixth grandchild for Carole and Michael Middleton and another little cousin for Prince George, Princess Charlotte and Prince Louis.

LONDON, ENGLAND - JUNE 04: Pippa Middleton during the Platinum Party at the Palace in front of Buckingham Palace on June 04, 2022 in London, England. The Platinum Jubilee of Elizabeth II is being celebrated from June 2 to June 5, 2022, in the UK and Commonwealth to mark the 70th anniversary of the accession of Queen Elizabeth II on 6 February 1952. (Photo by Chris Jackson - WPA Pool/Getty Images)

Chris Jackson - WPA Pool/Getty PIppa Middleton

Pippa and James tied the knot in May 2017 at St. Mark's Church in Englefield. Her royal nephew and niece, George and Charlotte, served in the bridal party.

A source previously told PEOPLE that Pippa and her husband have been looking forward to settling down and "having children and leading a quiet life."

Pippa officially announced she was pregnant in a column in Waitrose Weekend in the U.K., where she shared how the baby on the way impacted her exercise schedule.

"When I learned the happy news that I was pregnant, I realized I needed to adjust my four to five-day-a-week [fitness] routine," she wrote, noting that she kept her pregnancy a secret from her gym instructor until the "riskier months" were over.

Kate Middleton And Pippa Middleton

Karwai Tang/Getty Kate Middleton And Pippa Matthews
She also shared how fitness has been an integral part of her pregnancy.

"I've noticed my body change, but through exercise I feel it's being strengthened," she said. "I'm fanatical about sport and have looked at loads of books and websites on exercise during pregnancy, but have been disappointed by the limited technical information on what you can and can't do."