Prince William and Kate Middleton reportedly made no effort to introduce their children to their California-based cousin, Lilibet.
While speaking to US Weekly, royal expert Christopher Andersen revealed that there is still "a lot of tension" between the royal brothers after Harry resigned from royal duties.
He went on to say, "William and Kate made no effort at all to introduce Prince George, Princess Charlotte and Prince Louis to [Lilibet]".
Adding, "As far as I know, nobody believes they spoke [to each other]."
Despite the snub, the Cambridges sent their niece well wishes for her birthday, which she celebrated at Frogmore Cottage on June 4. The tweet read, "Wishing a very happy birthday to Lilibet, turning one today!"
Page Six reported the Sussexes invited Prince William, Kate and their three children to their daughter, Lilibet's first birthday but the family did not attend because of a royal engagement in Cardiff.
It comes after Angela Levin - an author who spent a year with the Sussexes while writing Prince Harry's biography - revealed she believes Prince Harry is "absolutely furious" over his treatment at the Platinum Jubilee.
She also alleged Prince Harry didn't even check what Prince William's plans were during the Queen's celebrations at the weekend, to properly arrange a meet-up between the two families before he and Meghan left on a private plane with their two children.
However, the Sussexes did visit the Queen and introduce her to baby Lilibet, who was born in the United States and hadn't yet met her famous namesake. It is also believed the couple was able to meet up with Prince Charles and Camilla.
"I think [Harry] would have been very, very upset that he was largely ignored. He still feels he's owed an apology," Levin said.
"But he's the one who should apologise. He said during the Oprah interview that Charles and William were trapped, he said he'd been cut off by his father.
"You can't just go around being rude about people and expecting them to open their hearts to you again," Levin said.
The Sussexes returned to California on Sunday night.