SISTERS IN SYNC

Pippa has been a rock to Kate since the princess was diagnosed with cancer

- NICI DE WET

IT WAS only the second time she stepped out publicly this year and as Kate, Princess of Wales, took her seat in the front row of the royal box at Wimbledon the crowd rose to their feet to give her a standing ovation.

Stunning in a purple dress from London-based luxury brand Safiyaa, a radiant, smiling Kate waved to the crowd. She looked visibly moved and her daughter, Princess Charlotte, beamed up at her with pride.

The other person by Kate's side at her surprise appearance at the Wimbledon men's singles final was the person sources have described as the future queen's closest ally - her sister, Pippa Middleton Matthews.

The 40-year-old is said to have been Kate's "rock" during the "incredibly tough" months of her cancer treatment.

"Pippa has been something of an angel and backbone for Catherine," an insider said.

"She's there at the drop of a hat, helping to pick up the slack on physical tasks when needed. But it's the emotional support she provides that is invaluable to Catherine." Their bond has been likened to the one shared by the late Queen Elizabeth and her younger sister, Princess Margaret.

"They are genuinely best friends who understand each other and [Pippa is someone] who Catherine can trust implicitly." The sisters have always been extremely close, and the bond they shared in childhood was carried through into adult life.

They lived together in a Chelsea flat in their twenties, Pippa was matron of honour at Kate's wedding, and they also shared the journey into motherhood.

But the 42-year-old princess' battle with an undisclosed form of cancer has seen their bond become unbreakable.

It has tested Kate both physically and mentally, and insiders says she's been leaning on her sister more and more.