Princess Diana's charm reportedly caught Prince Charles' attention even when he was still dating her sister.

Prior to dating the People's Princess, the Prince of Wales was seeing her older sister. In Katie Nicholl's book "The Making Of A Royal Romance," she covered how Princess Diana stole Prince Charles' attention.

"Charles had been paired off with Diana's older sister Sarah and they had enjoyed a skiing trip to Klosters," Nicholl wrote (via Express). "It was lissome and gamine Diana who caught the Prince's eye at a friend's barbecue in the autumn of 1980."

"The attentions of the Prince, who had been linked with a number of aristocratic suitors known as 'Charles' Angels' in the British press, was a novel experience for Diana, who had not year had a serious boyfriend," the royal expert added.

Princess Diana was only 19 years old when she and Prince Charles first met. The late Princess of Wales was teaching at a nursery school and was living in Earl's Court with three girl friends.

She reportedly immediately fell in love with the future king. The royals also approved of her as she was deemed to be the "perfect virgin bride."

There was no rift between Princess Diana and Lady Sarah McCorquodale, who previously dated Prince Charles. In fact, the older Spencer was proud to play cupid for the two. "I introduced them. I’m cupid," Lady Sarah said, following Prince Charles and Princess Diana's engagement announcement.

However, things immediately changed between Prince Charles and Princess Diana. In fact, according to body language expert Dr. Peter Collett, the heir was already showing signs of distress during their engagement.

"When Prince Charles and Diana announced their engagement there were already signs that all was not well," Collett said. "We can see that they together, but they are not in step and Charles is not adapting his behaviour to include Diana."

"In fact, if we apply the cut-out principle, and remove Diana from the scene, it does not look like anyone is missing," the expert continued.

Judi James added that the couple was showing signs of failed marriage when they exchanged "I do's." She noted that Prince Charles' "steepled brows" as he waited for his bride suggested that he was doing it for "duty" and the future king looked like he was suffering.

Prince Charles and Princess Diana
Princess Diana's being "lissome" and "gamine" caught Prince Charles' attention who was dating her sister at that time. Pictured: Prince Charles and Princess Diana at Westminster Abbey, London, for a centenary service for the Royal College Of Music on Feb. 28, 1982. Getty Images/Fox Photos