Samantha Markle, 53, criticised the newlywed's mother, Doria Ragland, of taking cash for an interview with Oprah Winfrey and dubbed Good Morning Britain host Piers Morgan a “pasty old troll”.
The Florida-based writer was not invited to Prince Harry and Meghan’s wedding on Saturday in Windsor.
After Piers questioned why Meghan Markle had not jetted to see her father following his heart attack, Samantha Markle attacked him on social media.
The GMB presenter previously said: “If my father was having heart surgery 8,000 miles away, I probably wouldn’t stay in London working out what songs I want Elton to play at my party..."
Sharing a link of the story featuring his comments onto her Twitter profile, Ms Markle wrote: “So @piersmorgan Morgan calls me a vulture,turns around slams my sister after slamming my father LOL I guess that Viagra is not working for your Piers.”
She added: “Everything about this man just screams Napoleon complex”.
Ms Markle, a former actress, also became embroiled in a Twitter row with some of her followers and slammed Meghan Markle's mother Doria Ragland for claiming she "cashed in" on her daughter’s royal status.
She said: “Even Doria cashed in talking to Oprah and some of the first photos out there only she could have had”.
She went on to reiterate her right to freedom of speech, adding: “Excuse me but I’m an American citizen with freedom of speech and I don’t take cues.
“We had a right to defend ourselves and speak out against nasty British media if no one likes it that’s just too bad”.
After a number of arguments with social media users, Ms Markle declared she was getting a massage, tweeting: “My haters are giving me a neck ache which is a really good excuse to get an Uber massage”.
Her latest Twitter rant comes after her announcement that she is in talks with Hollywood producers about turning her forthcoming book The Diary of Princess Pushy’s Sister into a major movie or mini-series for television.