will be celebrating her 20th wedding anniversary to on Wednesday and now her sister, Annabel Elliot, has made her feelings clear on the couple's relationship. The King and Queen , at Windsor Guildhall following a few tumultuous years .

But their love stood the test of time, with Camilla's devotion to her husband and the monarchy being particularly evident last year, when the monarch was diagnosed with a type of cancer, and she had to step up for him when he took a brief break to focus on his treatment. , has given a rare insight into the royal couple's love story.

Speaking to the , Ms Elliot, who is an interior designer and antiquities dealer, said: "They have known each other for a long time, almost their entire lives. Theirs, as any marriage should be, is a huge friendship."

The Queen first met the King sometime in the 1970s and they remained friends even during Charles's relationship and subsequent marriage to Diana, Princess of Wales.

Camilla shouldered much of the blame for King Charles and the late Diana Princess of Wales's marriage breakdown. Their relationship became official in 1999, and six years later, they married at a civil ceremony.

Speaking about Charles and Camilla's special bond, the Queen's sister said: "The key thing is that there is not really anything competitive between them.

"She is not trying to steal his thunder in any way at all. He is the King. And she will do her best to support him and be totally loyal to him.

"She is a very strong character and she sticks to her guns. It's quite extraordinary when I look back at how she has coped with everything.

"She is incredibly loyal to people she loves... She is a very straightforward person. You always know where you stand."

Ms Elliot added that another secret for their sucessful marriage is her sister's sense of humour.

She revealed: "They rather like quite a lot of things going wrong. Things going wrong on a tour, or something like that."

"I would almost say it's the biggest gift she has given him. The humour. It's been such a bond between them. And even if they argue, they will always laugh."