Katharine McPhee, 34, made a Parent Trap joke about shipping fiancé David Foster's daughters Erin, 36, and Sara, 37, off to Switzerland after the wedding

By Robyn Merrett
August 30, 2018 09:30 PM
Charley Gallay/Getty

Katharine McPhee is enjoying life as a stepmother-to-be!

On Thursday, the 34-year-old singer shared a photo from her Canadian vacation with fiancé David Foster, which caught the eye of Foster’s daughter Erin, 36, who commented, “CHIC.”

Following Erin’s compliment, Instagram account @commentsbycelebs captured a fan’s comment to the Barely Famous star, asking: “How awkward is it when your dad has a younger wife than you are?”

To which, Erin jokingly replied, “I’ve started saying I’m 31, so now she’s 3 years older than me. I’m a problem solver.”

Then, McPhee hilariously chimed in, writing, “@erinfoster stop blowing my age cover! the day we say I do, I’m shipping you and your sister off to Switzerland,” in reference to Erin’s 37-year-old sister Sara.

“Omg you’re gonna parent trap us!!! I knew it,” Erin replied.

Katharine McPhee

This isn’t the first time McPhee joked around with the Foster sisters.

On Tuesday, the American Idol alum posted a photo from Stuart Island, British Columbia, writing, “Country album drops this week.”

Moments later Erin, who seemed a little left out, commented, “FEELS LIKE I COULD HAVE BEEN INVITED ON THIS BEAUTIFUL TRIP.”

Instead of extending the invitation, McPhee humorously chided: “@erinfoster mommy and daddy need alone time.”

RELATED VIDEO: They’re Engaged! David Foster Proposes to Katharine McPhee During Romantic Italian Getaway

McPhee and Foster, 68, got engaged in Italy last month, the actress’ rep Leslie Sloane confirmed to PEOPLE.

“He did it at the top of this mountain in Anacapri. Totally dark, only stars,” McPhee explained in a text with Just Jared founder Jared Eng. “Thankfully he didn’t push me off the cliff. He said it was one or the other. And in the end he spared me.”

Erin and David Foster
Jason LaVeris/FilmMagic

The two first darting dating in 2017 and later confirmed their relationship when they stepped out together at this year’s Met Gala.

Before the big reveal, McPhee kept their relationship low-key telling PEOPLE she had “zero desire” to address relationship rumors.

“It’s great that they want to do that. I think it’s so amusing that people are amused by my love life,” McPhee said.

“Here’s how I look at it: I’m not doing anything wrong, so whether people think it’s true or false… there’s nothing bad happening in my life, so people can just say whatever they want,” she added.

McPhee and Foster have not yet revealed the wedding date.