Pippa Middleton is pregnant with her sister, who is only a few days pregnant from birth.
The Duchess of Cambridge's sister is scheduled to come in October to scan with her family and close friends after her 12-week week, the newspaper The Sun reported.
A friend who was close to the couple told the publication that Kate "could not be more delighted" when she heard that her sister, 34, was expecting her first child.
Duchess Kate is due to give birth to her third child at any moment and to form a family of five with Prince William, son George and daughter Charlotte.
The baby news comes almost a year after Middleton married James Matthews in May 2017.
"Of course, Big Sister Kate and her parents, Carole and Michael, were the first ones she told – James aside – and she could not have been more delighted, "said the source of the newspaper.  Pippa and the Duchess of Cambridge at their wedding last year. (AAP) "data-reactid =" 69 "/>
" It has been a tough couple of weeks for the two families, so this has some
Last month Middleton billionaire father-in-law David Matthews was charged with the historic rape of a minor in France
Allegedly, the 74-year-old has two historical acts in Paris and on the Caribbean island St Barts, where he owns a hotel and has been living for two decades.
Last year a complaint was lodged for the alleged attack. Matthews has denied all allegations.
"David Matthews categorically denies the claim and clearly denies the untrue and scandalous accusation," it said in a family statement.
© Nine Digital Pty Ltd 2018