David Foster's adult daughters have revealed the one problem they have with their father's wife Katharine McPhee.

The couple got married just over a year ago, making McPhee, 36, stepmum to Sara, 39, Erin, 37, and Jordan, 34 – as well as David's other children Alison, 50, and Amy, 46.

His daughter, Erin Foster, has hailed Katharine's influence on her 70-year-old father, insisting that their 34-year age difference has actually proven to be a positive aspect of their relationship — but she has shared one problem.

"Our biggest issue with (Katharine) is how hot she is," Erin teased to PEOPLE magazine.

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"Her body is insane. When she comes over for family Sundays, I'm like, 'Do not get in a bikini.'

My husband's always like, 'Kat, get in the pool!' And I'm like, 'Simon …'"

Erin said she believed the age difference has played a role in a positive way.

"Our dad comes from a generation where women are more passive. He is also passive about his feelings. Kat has really pushed him to open up.

"We could get into a fight, and then Kat's like, 'No, you have to pick up the phone, call your daughter back.' She's a facilitator, which is really important. She pushes him to be more emotional and more vulnerable."

Similarly, David previously described his 36-year-old wife as the "whole package".

The singer tied the knot with the 'Scorpion' star following a 12-month engagement, and he feels incredibly lucky to be able to call her his spouse.

He said of Katharine: "She's the whole package, you know? Singing, acting, beauty, she's got it all!"

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Shortly after their wedding, Katharine decided to take David's surname because she loves the "traditional" nature of taking a partner's last name.

She explained: "I'm a romantic. I've always liked the idea that it's still a traditional thing that exists in our society. I like the idea.

"I think it's romantic and it's a great last name ... We've been away since we've been married, so not [yet], but I love it. I just love the ring of it, so I'm enjoying it."