McPhee knows Meghan from their school days in Los Angeles, where they went to the same high school.
The source claims Foster and McPhee offered to host the celebrations at their home, but Meghan volunteered her and Harry's mansion.
Foster and McPhee got married just over a year ago, making McPhee, 36, stepmum to Sara, 39, Erin, 37, and Jordan, 34 โ 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 โ but she has shared one problem.
"Our biggest issue with [Katharine] is how hot she is," Erin teased to PEOPLE magazine.
"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 โฆ'"
"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, Foster 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 McPhee: "She's the whole package, you know? Singing, acting, beauty, she's got it all!"
Shortly after their wedding, McPhee decided to take Foster'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."
