'Grandma!' Erin Foster Teases Soon-to-Be Stepmom Katharine McPhee on Singer's Sexy Bikini Photo

Erin Foster left some love in the comments of soon-to-be stepmom Katharine McPhee‘s sexy bikini Instagram.

After McPhee, 34, who got engaged to Erin’s father David Foster, 69, this summer, posted a stunning shot of herself on holiday vacation in Hawaii wearing a vibrant yellow Onia bikini. And the Barely Famous star, 36, couldn’t hold back her cheeky comment.

“Grandma!!!! Wtf I don’t want to be next to you at the pool,” Erin said in a message first spotted by Comments By Celebs.

McPhee then hilariously poked fun at herself by replying, “@erinfoster Grandma carries it in the back. Don’t worry- you’ll see my cellulite when I stand up.”

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Since her father and McPhee went public with their relationship, Erin hasn’t been afraid to tease her dad about their coupling.

The comedy writer left a comment on her dad’s Instagram post from his vacation with McPhee to Italy in July, calling her soon-to-be stepmom, “Mommmyyy” (alongside a sea of heart emojis). David’s other daughter Sara, 37, also got in on the laughs, writing, “Out of the country. What did I miss?”

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But jokes aside, Erin couldn’t be happier to welcome McPhee into her family. During a Halloween bash, the star shared a photo posing with her father’s fiancé and her mom Rebecca Dyer — who was married to David from 1982 to 1986.

“Love my two moms ❤️,” Erin captioned the photo.

The couple opened up to PEOPLE about their journey to love in September. “We genuinely love and respect each other and have for a very long time,” McPhee said.

“She didn’t catch my eye on the first day because I was so busy trying to do my job,” added David. “But the reality is that in the last 12 years I’ve always had a connection with her.”