Erin, 38, was joined by her sister Jordan Foster and the Waitress star, 37, for a sweat session on Thursday.
She then posted a photo as the three of them held a plank and joked, "You ever side planked with your 37 yr old step mom? I have."
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Erin, Jordan and Sara sat down with PEOPLE in July 2020 and discussed how they felt about David and McPhee's marriage.
"Our biggest issue with [Katharine] is how hot she is," Erin told PEOPLE. "Her body is insane. When she comes over for family Sundays, I'm like, 'Do not get in a bikini.' My husband's always like, 'Kat, get in the pool!' And I'm like, 'Simon...' "
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She also joked that all of their husbands and Jordan's son Otto "hang on the side ogling her."
Other than that, the three women said McPhee — whom David met in 2006 when he served as a mentor to her and other contestants on season 5 of American Idol and married on June 28, 2019 — has only been a positive addition to their lives.
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"Interestingly, the age difference has played a role in a positive way," Erin said. "Our dad comes from a generation where women are more passive. He is also passive about his feelings. Kat has really pushed him to open up. We could get into a fight, and then Kat's like, 'No, you have to pick up the phone, call your daughter back.' She's a facilitator, which is really important. She pushes him to be more emotional and more vulnerable."
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David added that he thinks McPhee, with whom he welcomed son Rennie David Foster in February 2021, has navigated her stepmother role "really, really well."
"And so do they," the Grammy winner said of his daughters. "Because they could be horrible. It sure makes it easier for me that they get along. It's not a fleeting thing, either. It's been a few years."