Samantha Grant, who also calls herself Samantha Markle, told the Duke of Sussex to “think about the birthday wishes he never extended to his father-in-law”.
Ms Grant, who is estranged from Meghan, 37, and has not seen her in years, tweeted a message to Prince Harry on Saturday.
“Happy Birthday Harry! As you ‘mack down’ on your birthday cake, think about the birthday wishes you never extended to your father-in-law,” she wrote.
It was accompanied with a GIF of a hamster stuffing birthday cake into its mouth.
In a separate birthday message to Harry that was printed in the Daily Star, she reportedly urged Harry to “make his mother Diana proud” by “burying the hatchet” with his father-in-law Thomas Markle.
“Harry, how old do you have to get before you realise it’s time to bury the hatchet and be a gentleman and extend an olive branch to our dad,” she was quoted as saying.
“A birthday is a time to reflect on life and what is really important. Seize the moment to do the right thing.
“Not to mention the fact that William and Kate’s children will soon be reading about all of this.
“They will see every action and reaction. You’d be amazed what kids come across in cyberspace.”
Her latest remarks come after she labelled her estranged sibling the “DuchASS” as she took further umbrage to the new royal’s treatment of her father.
She lambasted her younger half-sister on Monday and stated that she should be “humane” to their father.
Their father did not attend the royal wedding after he claimed he was struggling with health problems following reports he had staged paparazzi photos in the run-up to the big day