Samantha Markle has accused her half-sister Meghan, the Duchess of Sussex, of being a narcissist and lying about their relationship during her interview with Oprah Winfrey.

Meghan and her husband, Prince Harry, spoke publicly for the first time about their departure from the royal family with Winfrey in a primetime TV interview that was watched by nearly 50 million people worldwide this month.

The two-hour interview featured numerous revelations, including that one member of the royal family questioned what colour skin their son might have.

In a deleted clip aired after the initial broadcast, Meghan was asked about her relationship with her half-sister, who has been vocal about her in the media, including writing a tell-all book called The Diary of Princess Pushy.

"It would be hard to tell all when you don't know me," Meghan told Winfrey at the time.

"I grew up as an only child and I wished I had siblings ... that's why I'm so excited to be pregnant so that Archie has someone.

"But the last time I saw her must have been 18 or 19 years ago."

Speaking to Heather du Plessis-Allan in her first New Zealand interview since the Oprah interview aired, Samantha Markle said those claims were inaccurate – and there is a photo of them together in 2008 at Samantha's graduation.

"Anyone who has a calculator or a functioning frontal lobe knows that was not 19 years ago. At her age, I would think she could do math, but I think the case is deceit more than inaccuracy."

Meghan also said Samantha changed her last name only when she started dating Harry, but Samantha told du Plessis-Allan that her name was Markle on her graduation certificate.

Samantha claimed her half-sister doesn't always tell the truth and the examples involving her made her doubt some of the other claims Meghan made.

"Shame on Oprah for enabling it."

Samantha and Meghan Markle at the former's graduation in 2008. Photo / Twitter

She recommended Meghan and Harry get counselling, and if they don't take that approach, the marriage won't last, she said.

Samantha said that she used to believe she and her father, Thomas, could reconcile with Meghan, but the interview reinforced that that likely won't happen.

"Do you forgive this person who has assassinated your character? Honestly, I don't think that's realistic, especially when you are dealing with a narcissist who is not capable or empathy or remorse or shame."

Nevertheless she still has hope, for her father's sake, that Meghan might try to make amends with him.

"If she did apologise, it probably wouldn't be sincere, but it would be great if she could at least give him that just so he gets some feeling of love."