The 68-year-old Canadian musician - who was previously married to Yolanda Hadid until 2017 - is set to tie the knot with the 34-year-old beauty, who is half his age, and the former '90201' has heaped praise on the 'Shark Night' actress.
The 37-year-old beauty said: ''She's great. She's a great addition''
David has seven children from previous relationships, but Sara hasn't ruled out the possibility of the couple giving her another step-sibling.
She said: ''You know I've learned to not rule anything out.
''I think anything is possible at this point. Who knows?''
Sara has been getting on well with Katherine - who is just three years younger than her - and they've even been for beauty treatments together.
She recalled: ''She's wonderful. The other day she was like, 'I got you a facial,' and like that's so cool.''
The record producer has been married four times before, and the former fashion model says her father is a ''real commitment'' guy.
She said: ''My dad loves love and my dad loves commitment.
''My dad's always been married. He's never been single really.
''He's a real commitment, marriage kind of guy and I'm happy for him.''
David's daughter Amy, 45, previously said she thinks he and Katharine are ''perfect for each other''.
She said: ''I know that some people have been hating on that relationship. All I can tell you is it absolutely works when you see them together. It's love. The haters [can] shut the f**k up because they are perfect for each other. I'm so close with [McPhee]. She's amazing and funny, fun, smart, elegant, timeless, beautiful, talented. I can't say enough about her.''
Whilst the family have welcome the news with open arms, David and his fiancée faced critics after announcing their engagement last month.
Taking to Twitter shortly after the news broke, Katherine wrote: ''y'all should be worrying more about registering to vote and midterm elections than who's marrying me. thank you for coming to my TED talk. (sic)''
A source previously said the couple have had a ''special bond'' from the start.
They said: ''From the very beginning, he and Katharine had a special bond. David took a lot of time off work to spend with Katharine. He is dedicated to his work, but he didn't want to miss out on any time with Katharine, so it was worth it for him. David's children are what is most important to him in his life, so the approval of his children was key to making this a lifetime choice. Katharine became friends with his kids and they are thrilled she is part of his life. She immediately hit it off with his children, and that is what sealed the deal for him. Their engagement was a beautiful thing. Everyone is thrilled for them and can't wait to celebrate.''
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