She also wants Meghan and Harry to apologise to Thomas and accused them of setting up a "smear campaign" against him amid the legal action Meghan is taking against British tabloid the Mail on Sunday for publishing a highly personal letter which she penned for her dad.
Speaking from Tampa, Florida, Samantha told hosts Eamonn Holmes and Ruth Langsford that Thomas is ready to testify against his youngest daughter.
"The truth is the truth. Based on that, of course he would testify firstly she had mailed him the letter. In the US if someone mails a letter and it lands in your postal box it's your property."
Samantha claimed that Meghan "shared [the letter] with several of her friends" and that "it went to the public forum".
"This claim of privacy doesn't seem to have a legitimate basis. It seems rather ludicrous," she said.
Speaking about Meghan's trips to two charities this week, Samantha said: "In my opinion, she is someone else and you know, when I saw that, it felt like a pr stunt.
"My first question was, 'how many coats and pairs of boots were the women at this shelter given?' 'What were the donations like?' and so, it still is my opinion that public events are helpful and they raise awareness.
"But, really people in desperate crisis situations need donations and help."