Their aunt, Pippa Middleton Matthews, has given birth to a boy. The newborn is the first child for Middleton Matthews and her husband, James Matthews.
Middleton Matthews, 35, is the younger sister of Princess Kate, 36, the mother of George, 5, Charlotte, 3, and six-month-old Louis.
Middleton Matthews gave birth to her son on Monday at the Lindo Wing of St. Mary's Hospital in London, the same hospital where Kate delivered all three of her children.
She delivered her son, whose name has not been announced, just a few days after she stood out in a dark green dress and heels at Princess Eugenie and Jack Brooksbank's wedding at Windsor Castle.
She announced the news of her pregnancy in June in the fitness column she writes for Waitrose Weekend, a magazine from the U.K. supermarket chain.
Middleton Matthew's pregnancy news came just a few weeks after Kate gave birth to Prince Louis in April.
She gave birth to her son on the same day Prince Harry and Meghan Markle announced they are expecting their first child.
Middleton Matthews married James Matthews, a hedge fund manager with Eden Rock Capital Management, in May 2017 with her sister and Prince George and Princess Charlotte, who served as page boy and bridesmaid, by her side.
The Matthews and their newborn live in London.