Thomas Markle wlll testify against his daughter Meghan in a lawsuit involving a British tabloid, the Duchess of Sussex's sister Samantha has confirmed.

Meghan launched legal action last fall against the Mail on Sunday and its parent company, Associated Newspapers, accusing them of "unlawfully" printing a private letter she wrote to her father after her wedding.

In its defence, the Mail on Sunday will rely on evidence from Meghan's estranged father, according to legal documents filed on Tuesday by Associated Newspapers and seen by The Washington Post. If the case were ever to go to trial, that raises the prospect of both Meghan and Thomas Markle being called to testify.

Markle's daughter Samantha, a half sister to Meghan, told the BBC on Wednesday: "If he is called, he will come."

Meghan and her husband, Prince Harry, are, of course, embroiled in a separate family drama, as they seek to "step back" from their royal duties. After an emergency family summit on Monday, Queen Elizabeth said on Monday that she had agreed to a "period of transition," during which her grandson and his wife would split their time between Canada and Britain. But she cautioned there remained "complex matters for the family to decide."

Meghan has been in Canada and was spotted on Tuesday at Vancouver's Downtown Eastside Women's Center, which posted a picture on Facebook with the caption "Look who we had tea with today!"

Meghan and Harry have complained about press intrusion, especially by the British tabloids, and it is thought that their sense of mistreatment by the media contributed to their decision to redefine their roles — and spend time outside of the United Kingdom. In an ITV documentary aired in October, Meghan said her British friends had warned her before marrying Harry: "The British tabloids will destroy your life."

That same month, the law firm Schillings filed a claim on her behalf in London's High Court, alleging the Mail on Sunday's publication of extracts from a handwritten letter she wrote to her father constituted a breach of copyright and an invasion of privacy.

The newspaper's defense includes the suggestion that Meghan had effectively breached her copyright, by allowing her friends to brief People magazine. In February, People spoke to five unnamed friends of Meghan's for a cover story: "Meghan Markle's Best Friends Break Their Silence: 'We Want to Speak the Truth.' " One of the friends acknowledged the existence of the letter Meghan wrote to her father.

The legal documents filed at London's High Court claim Markle "had a weighty right to tell his version of what had happened between himself and his daughter including the contents of the letter." The documents claim her father had the right to respond to the "one-sided and/or misleading account given in the People interview."

The newspaper's legal defence claims members of the royal family "generate and rely on publicity about themselves and their lives in order to maintain the privileged positions they hold and to promote themselves."

Parallels are being drawn to a 2006 case involving the publication of extracts from Prince Charles's private journals. The Mail on Sunday lost that case.

Mark Stephens, a lawyer specialising in media law, assessed that Meghan could probably win this one, but he wondered whether she may eventually drop it.

"I think chances are they can win some or all of this case, but at what cost?" he asked. "If a client wins but their reputation is damaged by cross examination, is that a win worth having?"

Stephens said her position was strongest on the copyright claim.

"She's got an absolute right to say: You can use my copyright, or you may not use my copyright," he said. "Now, the newspaper might say they have a right to reproduce a fair part of it, but I think they've exceeded what they needed."

The privacy question was less clear cut, Stephens said. The Mail on Sunday's position, he said, "was that Meghan had authorized briefings by other people, and that Thomas Markle was entitled to put the record straight. She could have more of a problem here."

Harry and Meghan have said they are funding the legal case privately and that any proceeds will be donated to an anti-bullying charity.