David Foster's Daughter Sees (His Wife-To-Be) Katharine McPhee's Bathing Suit and Calls Her "Grandma"

David Foster and Katharine McPhee, who are likely to tie the knot next year, are currently on vacation in Hawaii. The singer posted a requisite bikini selfie to Instagram, which drew a surprising comment from David's daughter Erin Foster.

“Grandma!!!! Wtf I don’t want to be next to you at the pool,” she wrote, according to PEOPLE.

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

These two aren't letting anyone forget their two-year age difference (Erin is 36, while Katharine is 34) ... with the continuation of earlier social media ribbing about the 34-year age gap between McPhee and her fiancé.

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

So now Katharine is officially "Grandma" to David's daughters, who all are around her age or even older, who approve of the forthcoming nuptials and who have previously called her Mommy on social media. In other words, the Foster family has jokes.

But seriously, Amy told PEOPLE earlier this year, "No one else needs to judge this relationship. I am amazed at why people believe that they have the right to judge, not just my dad’s relationship, but anyone else’s relationship… You don’t need to judge what you don’t understand. You’re not in it, you’re not living it. Specifically I was speaking to all of the really disgusting comments that I was seeing that were labeling her as some kind of gold digger. I was like, 'Are you serious?' She’s so successful in her own right. She could have anyone."