On her "Pratt Cast" podcast, which she hosts with Sarah Hyland's boyfriend, Wells Adams, Stephanie said Heidi is to blame for her fractured relationship with her brother.
"No one knows the real Heidi," she said. "Before the new 'Hills,' the 'Beginnings,' my mom, my brother and I all sat down and we said, 'We will not let this ruin our family again.'"
Stephanie recalled a recent situation where all the girls from the upcoming "The Hills" reboot were together. Heidi bought champagne for everyone but Stephanie. That, it seems, was the last straw.
"I'm done. I don't care. I'm not protecting them anymore," she said. "You have to put on your oxygen mask before you put on someone else's. … That's what I'm doing right now. If my mom can't handle this, that's her issue. I cannot help her."
Stephanie said she is done pretending Spencer and Heidi are "good people."
"They are the most toxic people I've ever met. They are Bonnie and Clyde till they die. … I'm going to expose them for who they are," she said. "If you ever have a son, Wells, I pray that he doesn't meet Heidi Montag."
Stephanie labeled her sister-in-law a "gold digger."
"If it weren't for Heidi, I would have an epic relationship with my brother. So would my parents," she said. "I've tried to be nice to her for 10 years because I love my brother."
Things weren't always this bad. When Stephanie and Spencer began filming the "The Hills" reboot, things were great.
"A few weeks into filming, [Spencer and I] started communicating more, just the two of us. It was like I was back in high school with him. I couldn't have been happier," she said. "It never changes. They flip the switch on me. … I do not have a brother and I never had a sister-in-law."