Katharine McPhee better start warming up her pipes.
When asked whether any of her musical friends would be contributing to the wedding, Erin, 37, explained that she'd love for her friends to perform -- especially her stepmother, 35.
"Yes, 100%," Erin said to E! News. "They're free, are you kidding? Right? It's free built-in entertainment."
Erin then revealed that she asked McPhee to perform.
"So Kat wrote me the other day, she was like, 'Is there anything that I can be doing to help you prepare?'" Erin recounted. "And I said, 'Yes. Vocal exercises. Prepare your voice because you will be performing.' She's like, 'OK.'"
Erin explained that she and fiance Simon Tikhman have a song from the Broadway musical "Waitress" that they're hoping she'll perform.
"She truly has such a beautiful voice," gushed Erin. "Simon and I are real fans of her so I'm very excited."
As to whether McPhee, who made her Broadway debut in "Waitress" last year, would actually perform is still up in the air.
"Well, they haven't said 'yes' (to performing)," said Erin's sister Sara of their dad, 70, and McPhee. "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."
Sara, 38, revealed that since she was young, Erin wanted Babyface, who worked with her father, to perform at her wedding. Unfortunately, the R&B star said he'd be unable to attend.
Sara and her family will be in attendance, however, including her 9-year-old daughter, who Sara hopes will join the wedding party.
"I think she's going to be a flower girl," said Sara. "I mean, she hasn't also been asked but I think she'll be a flower girl."