Samantha Markle has sent her half-sister Meghan a Christmas card urging her to "end the rift" with their father, according to reports.
The Sunday Mirror says Ms Markle has sent the Duchess of Sussex a card which asks her to give their father joy in the last years of his life by making contact with him.
Ms Markle and her father, Thomas Markle Snr, have made several public appearances and appeals to speak with his estranged daughter, who married Prince Harry in Windsor in May.
Mr Markle was due to walk Meghan down the aisle, but did not come to Britain after suffering health problems. He had also been found to have staged paparazzi pictures around his hometown.
The Sunday Mirror published a picture of the card which read: "Holidays are a time for family and are sentimental.
"Dad has been trying to contact you and is very hurt because you are avoiding him.
"Life is short and you know Dad is wonderful. Please give him joy in his last years by showing you love him. The time is now."
Mr Markle made another appearance on Good Morning Britain earlier this week, in which he said he hoped he and the duchess would be speaking again soon.
Ms Markle, a paternal half-sister of Meghan, told the Sunday Mirror of their childhood Christmases, spent watching Christmas shows together and helping to cook the dinner.
She insisted she loves her sister but said she does not agree with "the way she's behaving".
The Markle family say Meghan and Prince Harry have not spoken to them in six months.
While Meghan's father is not paid for his public appeals and interviews urging reconciliation with his daughter, Ms Markle was reportedly paid more than £1,000 per appearance when news of her half-sister's engagement and wedding emerged throughout the last year.
The card comes amid rumours of an internal Royal Family rift as well, with the Sussexes and the Cambridges said to have a "strained relationship" since the wedding in May.
All four will be at Sandringham House with the Queen on Christmas Day, though Harry and Meghan are understood not to be staying with the Cambridges at their home in Anmer Hall, two miles away, despite being there last year.
More from Meghan Markle
The Mail On Sunday reports pregnant Meghan and Harry will stay in the main home, despite it being a tight squeeze for all the guests.
Meghan's mother, Doria Ragland, is also said to have turned down an invitation to join the royals in Norfolk, opting instead to have Christmas in Los Angeles.