As ET previously reported, Samantha has been very outspoken about her sister and their family after not receiving an invite to the royal wedding.
Speaking to United Kingdom tabloid the Mirror, a source said the latest development "spoke volumes".
The source reportedly said: "She fully hoped they would let her in but she was turned away".
Samantha Grant - who has been using the name Samantha Markle in recent months - flew to the United Kingdom earlier this week with the aim of confronting the Duchess in person after being left off the wedding guest list in May this year. "It was a sad sight".
But on Sunday she blamed the Royals yet again for their behaviour towards Thomas Markle.
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Grant arrived in London more than a week ago with the intention of seeing her estranged sister, according to an interview she gave on the U.K.'s Channel 5 show with Jeremy Vine. Before Meghan's wedding, her father famously staged photos of himself reading a book on Britain and faked a heart surgery, while continuing to air his thoughts, frustrations, and private conversations with Meghan and Harry.
"I just think that families can be this way when there's confusion and when people are hurt".
"So moving forward I apologise and I wish things could be different".
According to the newspaper, the 53-year-old argued with police guarding the palace gates before handing over a letter for her half sister.
Since her arrival in the UK, Samantha has demanded an audience with her half-sister which has so far failed to eventuate.