Samantha Markle has been travelling in Italy, and the 53-year-old is determined to meet with the Duchess of Sussex before she returns home to the United States.
Her publicist Rob Cooper made a series of vaguely threatening tweets on Monday (local time), claiming that Ms Markle had tried to organise a one-on-one meeting with the Duchess, but her pleas had gone ignored by Kensington Palace.
"Samantha Markle is coming to the UK," Mr Cooper promised. "I currently have Samantha in Europe with just one short flight away."
He said his client wanted to discuss her 74-year-old father's health with her half-sister.
Thomas Markle has made countless headlines himself for his bizarre behaviour before and after his daughter's wedding to Prince Harry, and allegedly underwent heart surgery - although some have claimed he lied about the procedure.
Mr Cooper said the Duchess may not have been made aware of Ms Markle's attempts to meet with her, and that they had no other choice but to go public with their story.
In July, Ms Markle called Prince Harry a "wuss" on Twitter and accused Meghan's mother Doria Ragland of "cashing in" on her daughter's marriage.
Just weeks before making the comments, she had called for the estranged Markle family to reunite, a suggestion that went ignored by the Duchess.