More doubt has been cast on whether Meghan Markle's father will attend his daughter's wedding to Prince Harry on Saturday, as it is reported he is due to have heart surgery.
Thomas Markle had told US website TMZ he would not go amid a row over paparazzi photographs; then that he would; then that he could not, due to a planned heart procedure.
Kensington Palace declined to comment.
Ms Markle's estranged half-sister said he had faced an "unbelievable stress".
Speaking to Australia's Sunrise morning TV programme from Orlando, Florida, Samantha Markle said she was not sure if Mr Markle would be travelling to Windsor.
"He sent me a message that he was undergoing heart surgery," she said.
"We're all hoping he pulls through this now."
Meanwhile, Kensington Palace has announced Princess Charlotte will be among the bridesmaids and her brother Prince George will be a pageboy at the wedding this weekend.
Mr Markle became embroiled in controversy at the weekend, following reports that he staged paparazzi photographs of himself in the run-up to the wedding.
The pictures showed Mr Markle - apparently unaware he was being photographed - in a series of wedding-related activities, including being measured for a wedding suit.
On Monday, Mr Markle reportedly told celebrity news website TMZ that he would not attend the wedding.
TMZ later reported that Mr Markle had and wanted to attend - although he might not be able to because of health concerns.
And in a third report, the website said that the health issues and planned surgery would prevent him from attending after all.
On Tuesday evening, BBC royal correspondent Nicholas Witchell said Ms Markle wanted her father to be there, but her major concern would be for his wellbeing.
Kensington Palace has said events are "a deeply personal moment for Ms Markle".
"She and Prince Harry ask again for understanding and respect to be extended to Mr Markle in this difficult situation," the palace said in a statement on Monday.
By Jonny Dymond, BBC royal correspondent
One minute there's a perfectly scripted wedding about to reach its traditional-yet-modern, royal-yet-diverse climax in the St George's chapel at Windsor. The next minute there's twists and turns worthy of the finest soap opera.
It is difficult to believe that things are quite where the various Palaces involved in this wedding thought they would be at this stage.
It is of course conceivable that Thomas Markle could have heart surgery at today, somehow find the energy to make a transatlantic flight, meet the Queen, go to the wedding and walk his daughter down the aisle. But it seems a little unlikely.
So who will walk Meghan down the aisle?
Who will step under the standards of the Knights of the Garter, over the vault containing the remains of Henry VIII and Charles I and past 600 of years of Royal, English and British history?
Will it be Meghan's mother Doria, or will she walk alone? Now, there's a thought.
Mr Markle had been due to meet Prince Harry for the first time this week, as well as the Queen and the Duke of Edinburgh, before walking his daughter down the aisle at St George's Chapel in Windsor Castle.
Samantha, who has not spoken to her famous sibling in three years - and will not be attending the wedding - told Good Morning Britain the photographs had been done with "good intention" and not for money.
She previously admitted the pictures had been her idea, in order to portray a positive image.
The father of the ex-Suits actress lives in Mexico and is a former lighting director who worked on programmes including the 1980s TV show Married with Children and General Hospital, for which he and his team won two Emmy awards.
He and Ms Markle's mother, Doria Ragland, divorced when Ms Markle was six years old.
A brief history of TMZ
TMZ - which launched in 2005 - stands for Thirty-Mile Zone, an old Hollywood expression that describes the area where the studios are based and where most celebrities happen to live.
The US celebrity news site, which scooped the world's media on Thomas Markle's wedding plans, is no stranger to a-list exclusives (albeit not without controversy):
- Deaths - The showbiz site broke the news of Michael Jackson's death in June 2009 and was the first to report Prince had died in April 2016. But in 2008, TMZ was forced to remove an article wrongly reporting singer Miley Cyrus had died in a car accident and in 2013, it falsely reported rapper Lil Wayne was on his death bed
- Break-ups - In November 2006, TMZ revealed pop princess Britney Spears was to divorce her then husband Keven Federline and ten years later, broke the news of Brad Pitt and Angelina Jolie's split
- Babies - In September 2017, TMZ revealed reality star Kylie Jenner was pregnant and that Travis Scott was the father
Mr Markle, who has two children - including Samantha - from his first marriage, filed for bankruptcy two years ago.
Ms Markle has previously said: "It's safe to say I have always been a daddy's girl - he taught me how to fish, to appreciate Busby Berkeley films, write thank-you notes, and spend my weekends in Little Tokyo eating chicken teriyaki with vegetable tempura."
The wedding will take place on 19 May at 12:00 BST at St George's Chapel in Windsor.
With five days to go, Kensington Palace revealed that Ms Markle will spend her last night before getting married at a luxury Buckinghamshire hotel with her mother, Doria.
Prince Harry will be staying 15 miles away at the Dorchester Collection's Coworth Park in Ascot, with his brother, the Duke of Cambridge.