The 'Biology' singer chatted about bandmates Nadine Coyle, Nicola Roberts, Kimberley Walsh and Cheryl Cole while appearing on ITV's Lorraine on Monday (24 March).
"We spent 11 years together so it's almost like we're sisters in a way. We might not always see each other but we're always there to support one another in spirit or not."
Girls Aloud formed in 2002 on the TV show Popstars: The Rivals and were active until 2009 when they decided to take a hiatus to focus on solo projects. They reunited three years later for their ten-year anniversary, releasing a greatest hits album and touring, before bowing out.
Since then, Cheryl's been working on new music and has just rejoined X Factor as a judge, while Kimberley is pregnant with her first child. Nadine, meanwhile, gave birth to her first child in LA last year, while Nicola's been working on her Dainty Doll cosmetics line and Sarah's been gearing up for a solo concert at the end of the month.
Sarah added: "I think what makes a best friend is largely loyalty and longevity and just being genuine and being there for one another through thick and thin, no matter what."
She was speaking about friendship as she launched a new Model Mates competition which asks Lorraine viewers to nominate themselves and their friends to star in a glam photoshoot. Sarah will serve as a judge.
Watch Girls Aloud's video for 'Something New' below:
Got an iPad or iPhone? Reveal Mag is specially designed to fit! Find out more here