The late royal's former private secretary Patrick Jephson tells PEOPLE she still inspires her sons "along the difficult path they were born to follow"

It's been 28 years since Princess Diana's tragic death in a Paris car accident on August 31, 1997, but her influence on her sons, Prince William and Prince Harry, remains palpable. According

“We all remember the days when Harry and William were joshing with one another, and it all seemed set for their relationship and the future—that Harry, as Diana always used to say, would be William’s wingman,” Diana’s biographer Andrew Morton told People. “It’s a great loss for the monarchy.”