Meghan's half-sister today insisted if her mother was behind the letter to the Metropolitan Police accusing her father of child sex abuse, it was out of 'hatred and bitterness'.
Samantha Markle, 61, said her mother Roslyn Loveless - who died two years ago - was an alcoholic and 'not credible.'
It comes amid reports Roslyn was the one who wrote to the force in 2019 detailing allegations against Thomas Markle.
The pair were married from 1964 to 1975. Mr Markle has denied wrongdoing.
'If my mother wrote that letter then she should not be considered credible,' Ms Markle said.
'She was bitter that dad left her and had a successful career in Hollywood. There was a lot of hatred and bitterness there.
'She never ever once mentioned to me that she thought dad was a paedophile.'
Mr Markle, who is estranged from the Duchess of Sussex, was last week reported to the police by his and Ms Loveless's son, Thomas Jr, over alleged child sex abuse material found on computers.
Samantha Markle, the Duchess of Sussex's half-sister, has said if her mother did report her father over alleged child sex abuse, it was motivated by 'hatred and bitterness'
Roslyn Loveless, the first wife of Meghan Markle's estranged father Thomas, penned the letter which alerted police to allegations against him in 2019, it has been claimed
Ms Markle continued: 'My mother was a chronic alcoholic most of her life. She was verbally and physically abusive to me. When my parents were married we lived in Chicago because dad worked at a a TV station there.
'After my parents divorced I moved with my mom to Albuquerque, New Mexico while dad went to Los Angeles seeking work. Mom harbored a lot of bitterness towards dad. He became successful at ABC television and she was bitter about that.
'She grabbed my hair in a rage one day and yanked my head back. She stuck her face in mine and said she wanted to knock my teeth down my throat. I called dad in tears and he arranged for me to go to LA to live with him. I was 11 going on 12 and I could not wait to leave my mom's house because she was so awful.
'I went to live with dad in Santa Monica and a year or so later he met Doria (Meghan's mother) and she moved in with us.'
She added: 'We spoke on the phone every day during the last month of her life and she'd mellowed and we had a lot of heart-to-heart conversations.
'The worst thing she said about dad was that when she had me she wanted to buy a stroller and dad wanted to get a pool table instead. I think she would have told me if she believed anything like that (the child sex allegations).'
Meanwhile, last night Mr Markle told The Mail on Sunday he did not believe that Ms Loveless had sent the letter and said he felt the story was 'a set-up'.
Reacting to the idea that the letter may have been sent by his first wife, Mr Markle said: 'This shocks me. I'm not buying it. Roslyn and I broke up years ago but I was good to her in the divorce.
'I find it hard to believe she would write a letter like that. She was a really bad alcoholic. It's really sad but she would never have written something like that letter without someone making suggestions to her. She had really bad alcohol problems for years.
'I feel like this is a set up. I would like to see the letter Roslyn wrote. Roslyn and I were fine. She's dead so she can't speak out for herself. Why is this all coming out now? Why is my son trying to destroy me? Why is Meghan dumping on me?'
He said Ms Loveless cooperated with a 'really negative' story about him but he did not believe she would accuse him of being a paedophile.
'My first wife and I had an OK relationship for many years because of our kids. I don't believe she would say bad things about me. She's not around to defend herself. I think this was something that was maybe put in place to discredit me after my death. I'm alive and I am calling b*******.'
Mr Markle faces child sex allegations which he denies and told The Mail on Sunday he was 'not buying' the idea that his first wife, who died two years ago, wrote the letter
The 82-year-old last night confessed to this newspaper that he paid for sex in the Philippines but denied sleeping with underage girls.
And he vowed to 'do whatever it takes' to clear his name after being accused of sickening child sex crimes.
Authorities in the Philippines are investigating the former Hollywood lighting technician for allegedly hoarding child sex images.
The Met has also faced questions over why it sat on the letter which raised the alarm against Mr Markle for more than seven years.
Meghan initially said she learned of the allegations in 2018 after the letter was passed to her by Met detectives but the force has since said it actually alerted her a year later.
The duchess said she had been 'given assurances' at the time that the claims were being investigated.
She added the allegations against her father were 'more sickening than one could ever imagine', adding: 'My thoughts are with any children who may have been harmed.'
Mr Markle has strongly denied wrongdoing since The Sun revealed he was being investigated in the Philippines.
Child sex images reportedly held by Mr Markle were turned over to police last week by his son, Thomas Markle Jr.
The son claimed that he found child abuse images on computers his father left behind in the Philippines when he returned to the US earlier this year.
Mr Markle Jr told The Sun: 'When I found this material, I was horrified and I had no option but to report my dad. I have supported, cared for him and helped him for years, but that stops now. I won't speak to my father ever again unless I'm called to give evidence against him.'
Mr Markle, pictured with nurse Rio Canedo, is the subject of a child sex investigation after being reported to police by his son
Thomas Markle Jr, Meghan's half-brother, claimed to have found the child abuse material packed up in boxes at his father's Philippines home
Speaking to the Daily Mail on Friday Mr Markle called the allegations against him 'sick lies' and said he was 'very upset that my son is making these allegations against me'.
He was married to Meghan's mother from 1979 to 1987, with the couple separating when their daughter was around two years old.
Days after Prince Archie's birth in May 2019, Ms Loveless described her ex-husband as an 'awful father' who should never be allowed to meet his grandson.
She said Meghan 'would be a fool to have him in her life' and added: 'Repeatedly he has shown to Meghan he cannot be trusted. She must know that now. I speak not out of bitterness, my life moved on many years ago, but out of concern.
'Archie deserves to be brought up in a happy home, not in this vengeful situation that has been created. Tom was an awful father. What makes him think he'd be a great grandfather to Archie?'
At the time Mr Markle dismissed his ex-wife's comments as 'not valid'.