Clara McGregor, who is also an actress, posted a photo of herself holding her new sibling.
“Welcome to the world little brother,” she wrote on Sunday in an Instagram post on her verified account.
“Congratulations to my Dad & Mary — this is the greatest gift.”
Her sister Esther McGregor also posted a photo to the site. In her post she called the baby “Laurie.”
“Met my little brother looking like a pirate,” Esther wrote, referring to her own nautical-style outfit.
“I recommend! Welcome to the family little Laurie.”
Clara and Esther are two of McGregor’s four daughters from his marriage to production designer Eve Mavrakis.
The family are not the only Hollywood clan to have welcomed a baby during the pandemic.