Erin Foster wants to give her stepmother Katharine McPhee a special role at her upcoming nuptials: Wedding singer.

The 37-year-old TV personality, one of musician and record producer David Foster's five daughters, confirmed in August that she is engaged to Simon Tikhman.

When asked at the MIRROR Westfield Century City grand opening event on Tuesday if other members of the family, like Katharine, will perform at the wedding, Erin told E! News,  "Yes, 100%. They're free, are you kidding? Right? It's free built in entertainment."

"So Kat wrote me the other day, she was like, 'Is there anything that I can be doing to help you prepare?'" Erin continued. "And I said, 'Yes. Vocal exercises. Prepare your voice because you will be performing.' She's like, 'OK'...there's a song from Waitress that we really want her to perform that we really love. I don't know what else. Yeah, just anything. She truly has such a beautiful voice. Simon and I are real fans of her so I'm very excited."

Katharine, 35, made her Broadway debut as Jenna in the musical Waitress in 2018.

Katharine McPhee, Erin Foster

Charley Gallay/Getty Images for Bumble

"Well, they haven't said yes," Erin's sister Sara Foster, 38, said about their dad, 70, and Katharine possibly performing. "She might have to pay them to do that. They charge a lot of money to do things like that. So she is just assuming that they'll do it, but she hasn't asked formally."

"Since she was a little girl, she was like, 'I just want Babyface to sing at my wedding," Sara continued. "And obviously, my dad's worked with him and he's a family friend, and Erin was like, 'Oh, he's definitely- it's a lock.' And then he RSVP no to the wedding. So she's a bit depressed...I saw him recently. He's not coming. He's performing in Vegas. He's not canceling Vegas."

Erin and Simon are set to tie the knot on New Year's Eve.

"Well, I think a lot of people are excited to have New year's Eve plans because New Year's Eve sucks most of the time," Erin told E! News. "So when you have something that fun to celebrate and then also, it's everybody else's New Year's Eve, so they're going to be in a celebratory mood, it kind of makes the most sense. But it is hard for people with kids because they're taking family vacations at that time, so we've...had some people who feel like it's inconvenient for them, for sure."

"I'm honestly just not a very traditional person so I don't ever follow a rule of how things are supposed to go," she continued. "So I think we're just gonna kind of wing it. I'm getting married at night, I'm not gonna do daytime—I'm not doing photos before. We don't want to see each other before, well I guess that's kind of old school and traditional. But none of our photos will be in daytime and I don't really care."

Sara said her daughter Valentina, 9, will be part of Erin's wedding party.

"I think she's going to be a flower girl," Sara said. "I mean, she hasn't also been asked but I think she'll be a flower girl. Yup."

Sara also joked that Erin is "such a freaking bridezilla."

"You can't even say like one negative- you can't say like, 'Oh, it could be a little bit cold the day of your wedding,'" she said. "She's like, 'Why are you so negative about my wedding?!' I mean, you have to just, you can't say a word."

Sara is planning Erin's bridal shower and bachelorette party.

"My speech will be more like a roast, so we may never speak after, but she deserves it," she joked. "Nobody wants to listen to some love story for 20 minutes about your, the best sister in the world...No, no, no, no. We're going to talk about all the creepy s--t you did when we were kids."

Sara Foster, Erin Foster

Courtesy of Theory

Erin has been prepping for her big day with some dietary changes...in theory.

My workout routine hasn't really changed but I guess I'm trying to eat better," she said. "But I keep saying I'm gonna start eating better and then I just keep putting it off. And so I have six weeks left and now I'm like, OK, today I'm going to start eating better and taking my workouts more seriously."

"I've been more focused on skincare," she said. "I really wanna have glowing skin and have like my hair shiny. But here's what I'll say: I think that most brides don't look like themselves on their wedding day. They're really skinny. They're really stressed out. And to me, that doesn't look like a happy woman. I think that I want to look like myself on my wedding day. So I'm like, I'm having a glass of wine when I want to drink and I'm having a bite of cake if I want to have a bite of cake and like, I want to look like myself. I'm not trying to look like an unrealistic version of myself on my wedding day that I can't maintain."

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