Samantha accused the Duchess of Sussex of putting on "fake waves and smiles" in her most recent public rant, after it was revealed Meghan hasn't spoken to their father in months.
Meghan's relatives have continued to defy royal protocol by giving interviews after the Duchess' dad Thomas Markle pulled out of attending her wedding.
Thomas, who says he recently had heart surgery, has hit out at his daughter for cutting him out of her life since his mistake.
Samantha seethed on Twitter: "I am not candy coating anything anymore! The DUCHA** should be humane to our father who has given her everything… and this media c**p can stop!"
"He was the one who was always there for her. Fake waves and smiles can stop. The ducha** can bow to the daddy."
She added: "Sorry folks there is no valid or a good argument in favor of inhumane treatment of both sides of the family and a best friend of 30 years."
"Snobbery and arrogance are not humanitarian traits. She has an obligation especially to treat our father with kindness."
"She owes my father a lot of love and respect. No one wants to spoil her marriage and marriage is not at issue.
"What is at issue is the kind treatment of our dad."
Thomas has said the royals were freezing him out of his daughter's life over his alleged staged photos and controversial interview with a British talk show fronted by Piers Morgan.
He said: "My thing about my daughter right now is that I think she is terrified."
"I see it in her eyes, I see it in her face and I see it in her smile.
"I've seen her smile for years. I know her smile. I don't like the one I'm seeing now."
- 'That Fool': King Charles Trashed Prince Harry and Meghan Markle Over 2022 Netflix Documentary, Royal Insider Claims
- Meghan Markle Accused TWO Royals of Discussing Her and Prince Harry's Son's Skin Color, New Book Claims
- Kate Middleton 'Shivers' When She Hears Meghan Markle's Name, Sister-in-Laws Haven't Had 'Direct Communication' Since 2019
"This one isn't even a stage smile – this is a pained smile.
"That might be just having a couple of bad days. I don't know. It really worries me. I think she's under too much pressure.
"There's a high price to pay to be married to that family."
Former Hollywood lighting director Thomas, also dad to Samantha, 53, and Tom Jr., 51, said he watched Meghan's wedding on TV as he recovered from heart surgery.
We pay for juicy info! Do you have a story for RadarOnline.com? Email us at email@example.com, or call us at (866) ON-RADAR (667-2327) any time, day or night.
Get the exclusive celebrity scoop on all the stars you love before any of your friends by subscribing to our new podcast Straight Shuter below!