Sophie Winkleman, Lord Frederick Windsor's wife, gave birth to their second child, a daughter, on 16th January, according to an official statement released by the monarchy.
They also revealed the name they've chosen for their baby daughter - Isabella Alexandra May.
The statement said: 'Lord and Lady Frederick Windsor have this morning announced the birth of their second daughter, Isabella Alexandra May Windsor. The baby was safely delivered at 11:59 p.m. on 16th January 2016 at Chelsea and Westminster Hospital.'
The royal couple live in the US, where Lord Frederick Windsor's base is, but travelled home to the UK for Christmas and welcomed their daughter Isabella in London.
Frederick and Sophie are already parents to two-year-old Maud Elizabeth Daphne Marina Windsor, who was given her middle name after the Queen.
Lord and Lady Frederick Windsor at the christening of their first daughter, Maud Elizabeth Daphne Marina Windsor, at St James's Palace in 2013
Born on 15th August 2013, the Windsor's first daughter was the second royal baby to arrive in the Summer of 2013, after Prince George's arrival.
Maud's grandmother, Princess Michael, said at the time: 'We haven't met Prince George yet as we have been on holiday – and of course we've been preoccupied with the birth of our granddaughter – but we look forward to doing so at Christmas.'
Sophie also had Christmas in mind for the two cousins to meet, when the family of three was meant to travel home for the holidays: 'Perhaps we'll introduce the babies at Buckingham Palace for the Christmas lunch, that would be very sweet.'
Sophie with the Duchess of Cambridge in 2011
Unlike her baby sister, Maud was born in the US, at Ronald Reagan UCLA Medical Centre in Los Angeles.
Sophie, an actress best known for her role in Peep Show and Claudia Winkleman's sister, married Lord Frederick six years ago at Hampton Court Palace.
The lavish ceremony was attended by members of the royal family like Princess Eugenie, but also showbiz stars such as Bryan Adams.
Isabella's dad Frederick is first cousins with the Queen, which makes the newborn the 48th in line to the throne.
The couple at the Queen's Diamond Jubilee in 2012
Proud grandmother Princess Michael spoke recently of having her son and his family back home in the UK: 'My granddaughter was two in August and I can't wait to play with her. I'm besotted and there will be lots of games. I'm sure I'll be just as besotted with the second one.'
We bet George and Charlotte will also be delighted to have another cousin in the family to play with. Congratulations to the happy couple!