"They had a very different life," Erin said of Brandon and Brody.
While appearing on Kate and Oliver Hudson's "Sibling Reverly" podcast, the sisters recalled the "emotional turmoil" they experienced while watching their father raise their stepbrothers, Brandon and Brody Jenner -- the children of David's then-wife Linda Thompson and Caitlyn Jenner.
"Our dad was raising other children. He wasn't raising us. He was raising Brandon and Brody," Sara, 38, recalled. "So, for Erin, what weighed so heavily on her, was me. I was, like, dealing with my whole own emotional turmoil, which was watching my father raise other children. That was the thing that kept me up at night."
Five years after David divorced Sara and Erin's mom Rebecca in 1986, he married Thompson. Erin, 37, spoke about how she and Sara's childhood differed greatly from their stepbrothers'.
"We never lived with our dad, right, after our parents broke up," she said. "So, from the ages of 3 and 5 we were living with our mom, and our dad was living in this $20 million house with Brandon and Brody. It had the trolley, it had the pool and this thing."
"They had a very different life," she continued. "But, out in the world, people would be like, 'You guys are spoiled brats and you guys have it all and you're rich kids.' And we would be like, 'I mean, yeah, but we don't live there. We don't have the gate code. We don't have a room there. We're not really a part of that world.'"
Both sisters -- who described their childhood and their 20s as being in "survival mode" -- admitted they have "deep" resentment around the issue.
"Mine was less resentment towards them, it was more like, we're misunderstood out in the world, because we have this fake, spoiled lifestyle, but we're not even really allowed to be a part of it," Erin expressed. "So, we get all the backlash for it, but we don’t actually get the benefits."
David, now 70, has been married five times, has five biological daughters and seven grandchildren. After his split from Brody and Brandon's mother, David then was married to Yolanda Hadid. The couple divorced in 2017.
Earlier this year, he married Katharine McPhee, who is younger than both her stepdaughters but appears to have a great relationship with them.
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