The Girls Aloud star announced last month that she is expecting her first child with boyfriend Justin Scott and her former bandmate believes she will be quite strict in comparison to new mum Nadine Coyle, who had a baby daughter just a few weeks prior to Kimberley's announcement.
She said: ''We were all shocked that Nadine was the first because we thought it'd be Kimberley. I think Kimberley will be the bossy mum, whereas Nadine has that LA thing going on.''
While the 32-year-old star hasn't seen Nadine - who lives in Los Angeles with her NFL player boyfriend Jason Bell - since she gave birth, Sarah says the Irish singer is having a fairly easy time as a new mother.
She revealed: ''I haven't met her baby yet, but she's getting on very well, surprisingly! She's really laidback, and so is the baby - she sleeps through the night. She must have had a chilled-out pregnancy to have such a chilled-out baby!''
Sarah, who is in a relationship with DJ Mark Foster, admits she isn't ready for children herself as she wants to focus on her music career.
She added to new! magazine: ''I'm very focused right now and so is Mark. I'm not going to deny that I'm 32 - I'm embracing that fact. But my mum was in her late thirties when she had me and I think a lot of women are dominant in the workplace these days.''
The '12 Years A Slave' director will receive the accolade at the London Film Festival in October.
Critics from all over the world were asked to name the best movie of the past 16 years.