"We are not on good terms. We are not on speaking terms, I do not consider them my family," she said during a fiery episode of her Pratt Cast podcast with Wells Adams.
After saying things were "great" between the brother-sister duo, she added, "You want to know what happened? Heidi."
The breaking point was a photoshoot with the "Hills" cast for MTV. Pratt was ticked Heidi cracked open a bottle of champagne and shared it with every single person in the cast except for her. When she looked to her brother for help, he allegedly did nothing to support her.
"Whitney [Port], sweet Whitney is mouthing to me, 'This is weird. Is this normal?'" she continued.
While that incident was the catalyst for her angry attitude, Stephanie said her sister-in-law's behavior has put her off for years. The first version of 'The Hills' drove a major wedge between Spencer's entire family.
"Before 'The Hills: New Beginnings,' my mom, my brother and I all sat down and we said we will not let this ruin our family again," she said. That ceasefire lasted until last week. "I'm done, I don't care, I'm not protecting them anymore."
"I'm also not going to pretend that Spencer and Heidi are good people, like I was doing for most of the series for my parents," Stephanie continued. "For the [bleeped] they have done to me recently, I'm done. This is why I moved to London. I'm done. I cannot live near them. They are the most toxic people I've ever met. They are Bonnie and Clyde till they die, so if they say the sky is black, all of us are morons for saying it's blue."
"If you ever have a son, Wells, I pray that he does not meet a Heidi Montag," she continued. "What Heidi Montag does, she has played my family for money. When my mom said, 'What do you want from Costco, toilet paper, paper towels?' She said a treadmill."
Claiming she's "done" with them for good, Stephanie promised to "not pussyfoot around them" and said she's "going to expose them for who they are."
"I've never known someone to be so entitled and have so much gall," she went on. "This feels so therapeutic right now because I'm so f--king tired of protecting her."
Despite all the bad she had to say about Heidi, she couldn't bring herself to be as brutal when it came to Spencer. "My brother is in love with her. I understand that when you get married, your wife becomes the most important thing," she said. "I love that, I hope my future husband puts me before his family."
Instead, she accused Heidi of stoking the division in the Pratt family, saying she's the one who "eggs on the fights." She also claimed that her own parents don't see their grandkid, referring to Heidi and Spencer's son Gunner.
"She doesn't want anyone to have any connection to Spencer or to Gunner," she said. "Heidi does not want Spencer to have a relationship with anyone but her. And you know what? That's fine. I don't want a relationship with Spencer anymore. Gunner, yes, I will miss the fact that I'm going to miss out on the cutest, cutest, cutest, cutest little boy's early years but it's not worth it."
Stephanie said there's no way she and Heidi would ever be able to squash the beef, claiming her sister-in-law is "a fake person." She also adamantly denied her outburst was a "directive" of MTV or manufactured for the show.
"This is me actually exiting my family," she said, as she audibly choked up. "I've tried to be nice to her for 10 years because I love my brother," she said, saying she "couldn't have been happier" when they rekindled their relationship early into filming.
"They flipped the switch on me and I have no idea," she added. "Heidi is always #1 to Spencer, so you know what, Spencer's dead to me. I do not have a brother and I never had a sister-in-law."
She also claimed she wasn't the only one who's been having a problem with Spencer and Heidi.
"Heidi was horrific to me and all the girls at filming," added Stephanie. "I had to call them all the next day and apologize on behalf of my family. I had to call of them the next day to do damage control because they were so horrible."
"The Hills: New Beginnings premieres June 24 on MTV.