Photos captured Samantha - diagnosed with multiple sclerosis in 2008 and is now wheelchair-bound because of the condition - being wheeled dejectedly away from the security office at the palace.
But her attempts to meet the Duchess of Sussex face-to-face were stalled when a security guard refusing to let her pass.
But, on Sunday, Samantha teased her Twitter followers, writing, "Everybody would love to know what's in that letter".
The insider added, "This is the closest Meghan has come physically to that side of her family since she married into the royal family but palace staff had clearly been instructed not to allow her access".
Moments later, Samantha was photographed at a souvenir stall buying novelty masks depicting Prince Harry and Meghan. Thomas did not attend the nuptials due to a heart attack and following public criticism over paparazzi pics of himself that he had had staged for money.
"There is so much water under the bridge and so much has spun out of control that was never meant to", Samantha told Vine. "He would have to reach out to her and I think their relationship became a lot more distant then and that hurt his feelings".
It is the latest in a string of high profile public bust-ups with the palace after an on-going family feud involving her half-sister and father, Thomas. However her arrival at palace gates has got her no further in her quest to speak face-to-face with Meghan it would seem.