Sara Foster teases 'mom' Katharine McPhee over bikini photo: 'Dress age appropriately'

Sara Foster, 37, is having fun with her future stepmom — who happens to be three years younger — singer Katharine McPhee, 34.

The former 90210 star Instagram-teased McPhee, who is marrying her father, producer and composer David Foster, 69.

When McPhee wore a bikini during a photo break on their Hawaii vacation, Foster posted the photo and wrote, "When your mom refuses to dress age appropriately." 

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Even McPhee had to appreciate the jest, writing in the comments: “I can’t even think of a good response.”

“As long as you keep looking this good in a bikini, I will be writing captions like these," Foster wrote back.

McPhee got into the fun as well, writing, "Not just a regular mom," when she posted the same photo.

David popped the question to American Idol standout McPhee during an Italian vacation in July.

Erin Foster, 36, made an Instagram quip last week when McPhee posted a photo of herself in a yellow bikini.

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