Here's what David Foster's daughter, 35, calls Katherine McPhee, 34 after engagement bombshell

The 35-year-old Erin Foster is taking the news of her 68-year-old father's engagement to a girlfriend half his age in her stride and poked fun on social media.

David, a musician, and composer, proposed to the 34-year-old Katherine McPhee on a mountaintop while the pair was on vacation in Italy, as can be seen from her conversation with a good friend.

Soon after the news went public, David shared on Instagram a photo of him and Katherine. In the caption, he simply wrote 'Yup!' and added a musical emoji.

Erin couldn't contain herself and wrote in the comments 'Mommmyy' with three heart emojis. It was a jestful reference to Katherine who is one year younger than her fiance's daughter!

Not to be outdone, Erin's 37-year-old sister Sara Foster simply replied: 'Out of the country, What did I miss?' Read more on our Twitter, @amomama_usa.

Both his daughters clearly support their father, and Sara exclusively told US Weekly before Christmas that they would 'be vocal' if they didn't approve.

"We just want someone that’s going to treat him well, and that cares about him, and I think he’s with someone right now that really does.”

Sara Foster, US Weekly, December 15, 2017.

She added that it "bothers [her] that anybody would say anything negative about it," and said it's nobody else's business if two people are happy together.

Sara also defended McPhee by reminding the haters that her father's girlfriend is a grown woman in her thirties.

Katherine and David have been dating for more than a year and made headlines when they first appeared in public at the Met Gala back in May 2017.

As reported by ET Online, this will be Foster's fifth marriage and McPhee's second. She divorced from her husband of eight years, Nick Cokas, in 2016.

Foster was previously married to B.J Cook, Linda Thompson, Yolanda Hadid and Rebecca Dyer, who is Erin and Sara's mother.