Erin Foster Pokes Fun at Her Dad After Katharine McPhee Engagement

Erin Foster couldn't help but poke fun at her father, David Foster, about his engagement with [...]

Erin Foster couldn't help but poke fun at her father, David Foster, about his engagement with singer and TV star Katharine McPhee.

The 25-year-old comedy writer left a comment on her dad's Instagram post Tuesday, calling McPhee, 34, "Mommmmyyy" along with a multitude of heart emojis.

(Photo: Instagram/Comments by Celebs)

Her comment, which was first captured by Comments by Celebs, came as David and McPhee confirmed the news that they will be tying the knot — a relationship status change that took place 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?"

This is far from the first time Erin has made light of her father and McPhee's age difference.

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 denied the romance with McPhee to PEOPLE — with another insider explaining 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 was not denying the relationship, opening up in October to what she really thinks about the former Scorpion star.

"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."

This will be the fifth marriage for Foster, 68, and the second for McPhee. Foster was previously married to B.J. Cook, Rebecca Dyer, Linda Thompson and Yolanda Hadid, while McPhee married Nick Cokas in 2008, with her rep sharing in 2014 that the Smash star had filed for divorce.

McPhee and Foster have been dating since last year and while they have not spoken much about their relationship, they have been photographed together several times. In May 2018, the pair made their red carpet debut at the Met Gala in New York City.

"When Kat talks about David, her face lights up. She's really so happy. They're actually so sweet together," a source told Us Weekly in November 2017. "They aren't hiding it anymore... David is really charming. The age difference might seem substantial to some, but they are really cute together and it makes sense."