David Foster and Katharine McPhee, who got engaged in 2018 after spending most of the year hiding their couplehood, are ringing in the new year on vacation with family in Hawaii, where two of David's daughters have been essentially ribbing Katharine on their social media accounts for being their future young and gorgeous mother — and grandmother! Sara and Erin Foster are both even older than Katharine, who is 34, which makes their teasing extra cute.

"When your mom refuses to dress age appropriately," daughter Sara, age 37, captioned a bikini shot of herself and Katharine, outside at Four Seasons Resorts Lanai on Instagram.

McPhee responded in the comments "I can’t even think of a good response" to which Sara replied "as long as you keep looking this good in a bikini I will be writing captions like these."

Meanwhile “Grandma!!!! Wtf I don’t want to be next to you at the pool,” Sara's sister Erin, 36, captioned another photogenic bikini shot of Katharine's that was taken last week.

“@erinfoster Grandma carries it in the back," Katharine replied. "Don’t worry - you’ll see my cellulite when I stand up.”

Clearly her future stepdaughters are just welcoming her further into the family fold with their social media comments, and she's in on the joke.

"My biggest realization of the week is that i’m going to have the grandkids call me Nanny McPhee," Katharine tweeted in October in reference to David's seven grandchildren.