Britain's Duchess Catherine has been giving her sister tips on working in America.

Pippa Middleton reportedly infuriated royal officials when she was pictured next to a friend waving a gun as they drove through Paris in April but she and her sister - who were pictured at the Wimbledon tennis final together earlier this month - are now said to be closer than ever and Catherine wants to do whatever she can to help her sibling boost her image.

A source close to Catherine - who was known as Kate Middleton before she married Prince William - to LOOK magazine: "Kate clearly wanted to show that Pippa is truly back in the royal fold.

"The pair have been spending more time together and Kate's even been giving her some tips for her promo tour around the USA when she launches her party planning book there in October."

It was previously claimed that senior royals felt Pippa wouldn't be able to promote her book, 'Celebrate', in case it embarrassed her sister.

A source said recently: "She would normally be expected to appear on every chat show under the sun.

"I know she won't do anything that she hasn't run by the duchess' advisers, but it is still a very tricky situation."

Despite being offered the chance to have the tome ghost-written, Pippa - who rose to fame as her sister's maid of honour at her April 2011 wedding - has insisted on writing it herself.

The source added: "She was offered a whole host of ghost writers but said no to them all and is very proud that it is all her own work.

"I just don't think she appreciated how much work it would take and may need some help from her editors."