'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.”