The couple shared a gorgeous black and white picture of Harry and Meghan with their son Archie taken during a family shoot to announce their pregnancy on Valentine's Day.
WATCH: Harry and Meghan welcome baby daughter and name her Lilibet 'Lili' Diana
The caption read: "We are all delighted by the happy news of the arrival of baby Lili. Congratulations to Harry, Meghan and Archie."
The post received more than nine thousand comments, the most since the couple marked their nephew Archie's birthday earlier in May, and a huge number of fans praised the couple for their "kindness" and for being "class acts".
The Duke and Duchess of Cambridge's message to the Sussexes
"So happy to see The Duke and Duchess of Cambridge take the high road and acknowledge this," one wrote, whilst another added: "Your kindness fascinates me Your Royal Highnesses."
"I never expected less than graciousness from the Duke and Duchess of Cambridge," remarked a third. A fourth wrote: "Class acts, Cambridges."
Prince Harry and William's relationship has suffered since the Sussexes announced they were stepping back as senior royals last year.
The Cambridges posted a lovely message to mark Lilibet's birth
The rift between the siblings and Harry's tension with his father Prince Charles has been widely reported on, but Harry has said how he's hoping to "heal" his relationship with his dad, and how he'll always love his big brother.
"I love William to bits, he's my brother, we've been through hell together, we have a shared experience, but we were on different paths," Harry told Oprah Winfrey in his interview in March.
The royals are indeed on different paths – living across the world from each other with William destined to be King one day, while Harry embraces his new freedom in the US.
But the first steps towards reconciliation were seen during Prince Philip's funeral in April, when the brothers were spotted chatting together in a scene that looked like old times.