Britain's Prince George is "very clever and articulate".
The two-year-old prince enjoyed a playdate with Maud Windsor - whose grandfather is Queen Elizabeth's cousin Prince Michael of Kent - before the birth of his sister Princess Charlotte last year and impressed his relatives with his advanced development.
Maud's mother, actress Sophie Winkleman - who is officially titled Lady Frederick Windsor - said of Prince William and Duchess Catherine's son: "We were invited to tea at Kensington Palace just before Princess Charlotte was born.
"Maud and George got on very well. He's a very clever, articulate little boy and was speaking long before other toddlers his age."
Lord Frederick and Sophie welcomed their second child, Isabella, into the world on January 16 and the pair are thrilled to have two daughters.
Financial analyst Frederick, who is known as Freddie, told Britain's HELLO! magazine: "I am thrilled to have a second little girl.
"I didn't realise how funny little girls can be until I had one. I'm also thrilled they will have each other.
"As a child I had - and still have - a close bond with my younger sister [Lady Gabriella Windsor]and it will be wonderful to watch Maud and Isabella play and grow up together like we did. The bond you form with your sibling can be the closest one you have with anyone in your life."
The couple had been living in Los Angeles since they wed six years ago but are happy to have returned to the UK to raise their daughters.
Sophie said: "We had a fascinating, fun time in LA.
"The place is exciting when you don't have any responsibilities. But when you're suddenly in charge of a little person's life, that changes."