The 35-year-old comedy writer and Barely Famous star left a comment on her dad’s Instagram post on Tuesday, calling McPhee, 34, “Mommmyyy” (alongside a sea of heart emoji, natch).
Her comment, first captured by popular Instagram handle Comments by Celebs, came as David and McPhee confirmed the news of their engagement on social media — which happened during their vacation to Italy.
David’s other daughter Sara, 37, also got in on the laughs, writing, “Out of the country. What did I miss?”
It’s not the first time Erin has poked fun at David and McPhee’s coupling.
Back in September, as rumors of their romance popped up, Erin shared a photo of the two to her Instagram Stories alongside the captions, “Excited about my new step mom,” and “My parents.”
At the time, a source close to David was denying his romance with McPhee — and another insider explained that “Erin’s quite obviously tongue-in-cheek and sarcastic Instagram comments were pointed at those who can’t seem to grasp that her dad and Kat can be colleagues, collaborators and longtime friends without it being some big romance.”
“They can’t even perform together without people saying they’re headed down the aisle,” the source said. “Erin was messing with people who jump to conclusions.”
But Erin herself wasn’t denying it. A month later, in October, she opened up about her dad’s then-girlfriend on the red carpet of the BLEUSALT Launch Celebration in Malibu.
“She’s amazing,” Erin told PEOPLE of McPhee. “I really like her. She’s awesome.”
She continued, “My dad is kind of a hopeless romantic, and Katharine’s awesome. We really, really like her, and we’ve known her for a really long time. And if he’s happy, we’re happy.”
David and McPhee first sparked romance rumors in May 2017 when the two were spotted enjoying a PDA-filled dinner date at Nobu in Malibu. The pair was also said to have traveled together (with McPhee’s mom) not soon after on a Vancouver getaway.
Before finding romance, the two had a long friendship, with David first meeting the American Idol alum on the set of the hit reality singing competition when she was a contestant in 2006.
McPhee previously collaborated with the Grammy-winning producer for a few live performances, including his 2008 PBS tribute concert Hit Man: David Foster & Friends.
They eventually went public with their romance when they stepped out together at the 2018 Met Gala.
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She was married to Nick Cokas, though the two divorced in 2016. David was in attendance at the couple’s 2008 wedding, playing piano when Cokas sang Billy Joel’s “She’s Got a Way” and McPhee returned the favor with a rendition of “Unforgettable” by Natalie Cole.