David Foster's daughter defends composer's 34-year age-gap with wife Katharine McPhee... despite being older than her own stepmother

'I think they've gotten so much s*** [for] the age difference,' the 45-year-old socialite told Let's Be Honest with Kristin Cavallari on Tuesday. 'Everyone has an opinion about something. And you know what? They're like 12 years in and they are happy'

Sara continued: 'She's a fully consenting lady. This is not like some child bride. She is in her 40s'

The fashion designer (R) is three years older than McPhee, who sweetly supported her and her younger sister Erin (L) at the launch of their Nashville clothing boutique, Favorite Daughter, on June 5

'She's really amazing. I have to tell you she really is. It's not easy coming into this family,' Sara said. 'I am older than her...I look at her as a sister but, like, she is my stepmom. She's my dad's wife'

The Spencer Pratt supporters are proud parents of five-year-old son Rennie David, and Foster has five older daughters from past marriages (pictured May 7)

McPhee (3-L) famously met 'guest mentor' Foster (L) when she competed on the fifth season of the former Fox singing competition American Idol in 2006 where she placed runner-up 

The Los Angeles native and the Canadian grandfather-of-seven are next scheduled to perform together on July 10 at Kresge Auditorium inside the Interlochen Center for the Arts in Interlochen, Michigan (pictured June 12)

In February, McPhee wrapped her role as a grieving wife in Joshua Caldwell's abduction thriller Three People in the Woods alongside Henry Thomas and Jim Gaffigan