Sara, Erin and Jordan Foster only have one problem with their father David Foster’s wife, Katharine McPhee.

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

In an interview with People magazine, it was revealed that Foster’s daughters’ big problem with the “American Idol” alum is her beauty.

“Our biggest issue with [Katharine] is how hot she is,” Erin teased. “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…'”

However, it’s not just Erin’s husband, Simon Tikhman, that is drawn to McPhee, as Sara revealed that all of their husbands “hang on the side ogling her.”

“Even my 2-year-old son [Otto],” Jordan jokingly added.

Otherwise, the women say, the former “Smash” star has been a great addition to their lives, even when it comes to the age difference between her and David.

“Interestingly, the age difference has played a role in a positive way,” Erin explained. “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 added: “She’s a facilitator, which is really important. She pushes him to be more emotional and more vulnerable.”

Similarly, Jordan noted that David had been with women his own age “forever,” which has resulted in several splits.

“Age-appropriate hasn’t worked in the past,” said Jordan. “So I think this is fine.”

David also said that he’s not concerned about the age difference because he and McPhee have so much going for them, despite the “haters” who “are always going to be haters.”

“But, the way I’ve resolved it, there’s 10 things that can bring a marriage down. There’s infidelity, there’s geography, there’s finances, there’s kids …” the megahit producer explained to the outlet. “There’s so many things that can bring a marriage down. Only one of the 10 is an age difference. So if we feel like we have the other nine right, that we’re really solid on the other nine, the one is not going to break us up.”

He also offered his wife praise for coming into his family and getting along with all of his grown kiddos.

“I think she navigates it really, really well. And so do they,” David gushed. “Because they could be horrible. It sure makes it easier for me that they get along. It’s not a fleeting thing, either. It’s been a few years.”

Amy also noted that the age difference isn’t apparent in the relationship, noting that David and McPhee stayed with her after she had a mastectomy.

“Kat is not a trophy wife — she has a lot to say, she’s interesting, she has a fantastic career and she’s nobody’s trophy,” Amy explained. “You don’t even think about the age difference when they’re together because they both are emotionally the same age, which is either 36 or 70, I don’t know. Sometimes my dad is super young and sometimes Kat is super old.”

Now, Erin said that she and her sisters are just glad to see their father happy.

“We’ve been through a lot with each other, and we’ve arrived at a place in our lives where we’re all happily in our own relationships. We want our dad to be happy in his relationship,” she told People. “We are not dating our dad so we are not competitive with the woman who comes into his life. We want him to be happy, and it doesn’t matter if she’s our age or if she’s his age because we just want him to be with a good person.”

Erin continued: “And the truth is, ironically, it’s much easier for us to get along with someone who’s our age because she’s similar to us. She’s someone who we would be friends with.”

Sara also noted that McPhee “pushes [David] to be a better person.”