Lifestyle Style 'Grandma!' Erin Foster Teases Soon-to-Be Stepmom Katharine McPhee on Singer's Sexy Bikini Photo Erin Foster left a cheeky comment on her soon-to-be stepmom Katharine McPhee's Instagram photo By Kaitlyn Frey Kaitlyn Frey Instagram Twitter Assistant Style & Beauty Editor, PEOPLE People Editorial Guidelines Published on December 27, 2018 10:24 AM Share Tweet Pin Email 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.” Looking for more style content? Click here to subscribe to the PeopleStyle Newsletter for amazing shopping discounts, can’t-live-without beauty products and more. Since her father and McPhee went public with their relationship, Erin hasn’t been afraid to tease her dad about their coupling. Rob Latour/Variety/REX/Shutterstock 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?” RELATED PHOTOS: The Best Celebrity Engagement Rings of 2018 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.”