A series of photos of Prince George and his sister Princess Charlotte have been released by Kensington Palace.
The four pictures show the princess being cradled by her elder sibling as they sit on a cream couch at Anmer Hall in Norfolk, their family home with the Duke and Duchess of Cambridge.
The pictures were taken by Catherine just weeks after Princess Charlotte was born on 2 May.
In an earlier tweet, Kensington Palace said the images were "very special".
In each of the pictures, Charlotte is lying on her brother's lap, wearing white. The prince is wearing a white shirt, blue shorts and blue socks.
In one, he appears to be kissing his sister on the forehead.
The Duke and Duchess of Cambridge and their children have been at Anmer Hall since leaving London shortly after the birth of the princess.
Prince William and Catherine are expected to be based there for the next few years as they raise Princess Charlotte and Prince George, who is almost two, with the help of a full-time nanny.
Photography is listed as one of the duchess's hobbies in her biography on the royal website. The pictures were taken on a Canon EOS 5D Mark II camera.
The first official images of Prince George, released in 2013, were taken by the duchess's father, Michael Middleton, in the garden of their home in Bucklebury, Berkshire.
Commentary: BBC Royal Correspondent Peter Hunt
The photographs provide a rare glimpse of, as things stand, the future of the British monarchy.
Such glimpses will remain reasonably rare as these royal children grow up.
They'll next be captured together when Princess Charlotte is christened in July.
In all of these four images the baby princess - just a few weeks old at the time the photos were taken by her mother - is in the arms of her brother, Prince George.
As the toddler destined to be king continues to adjust to having a new addition to his family, his parents will hope the 22-month-old and the one-month-old will forge a bond as strong as the one enjoyed by Princes William and Harry.
Harry has spoken of how they understand each other and give each other support.
His older brother told someone recently that George was very lively and Charlotte had given her parents a "few sleepless nights".
On Friday, it was announced Princess Charlotte will be christened on 5 July.
The christening will take place at St Mary Magdalene Church at the Queen's Sandringham estate.
Princess Charlotte was born on 2 May at London's St Mary's Hospital, weighing 8lbs 3oz (3.7kg).
The fourth in line to the throne, her full name has been registered as Her Royal Highness Princess Charlotte Elizabeth Diana of Cambridge.