Kate and Pippa
Sisters act: Kate and Pippa Middleton spent their New Year's eve at a common friend's place in Bucklebury.Reuters

Though details of the New Year celebrations by Prince William, Kate Middleton and Prince George have not been revealed, Kate is reported to have opted for a quiet evening, without her husband and son.

The Duchess of Cambridge spent her first New Year's Eve as a mother with her younger sister, Pippa Middleton.

The Middleton sisters were spotted at a mutual friend's home in Bucklebury, where Catherine joined Pippa for a laid-back party on the afternoon of 31 December.

According to guests at the party, Kate looked relaxed away from the royal spotlight and she seemed quite happy.

"Kate looked incredible. She was very jolly and very happy," a partygoer told People magazine, adding: "The thing that really strikes you is that she is just so normal and smiley. There are no lights on Kate here, so she can be totally relaxed and herself."

The source also said that the 31-year-old mother of five-month-old baby has lost almost all her post pregnancy weight and is back into her slim shape. Kate gave birth to Prince George in July.

"[Kate] has really got her figure back. If anything she actually looks better now than before she had Prince George," the partygoer added.

"I came away from the party thinking, 'She looks really happy.' Kate really is glowing and full of life."

The Duke and Duchess of Cambridge spent their first Christmas with Prince George at the Queen's Sandringham estate along with rest of the royal family members.

In 2013, William and Kate had spent their holidays with Kate's parents, Carole and Michael Middleton.