"They're so cute; how can you say no to that?"

IT WAS rapping superstar Iggy Azalea's comment that cemented Gympie's sweet sister singing duo The Dennis Sisters a place on the X-Factor's next round when their soulful audition on the hit show aired on Channel Seven last night.

The sisters: Biannnah and Tiana Dennis, 15 and 16-years-old delivered a touching rendition of Miley Cyrus' Butterfly Fly Away on episode six of the reality show that has launched the careers of a number of Aussie household names, including our very own country music success Caitlyn Shadbolt.

Vying for a place on the live shows, the sister act was sent to bootcamp (the next stage) with three yes' from judges and a bag full of praise, but a warning from judge Adam Lambert of US screen and stage fame that their "harmonies need a lot of work."

Experienced x-factor judge and ARIA chart-topper Guy Sebastian, who the camera caught with a huge grin of admiration when one of the sisters hit a high note, said the performance was beautiful to watch and very emotional for him.

"It's what music's about - there was just this authenticity to it and even though it wasn't amazingly perfect at times, it was honest and it represented this unity," he told the sisters.

"I felt like you carried each other and I think that's beautiful.

"It's what sisterhood's about," he said before he gave them the green light with a '100% yes' to move forward in the show.

Aussie rap sensation Iggy Azalea, who helped persuade Adam Lambert to pass the girl's through, was wowed by the sister's performance considering their young age.

"You guys individually have really great voices.  I was really surprised how together you were because you guys are young."

"I love the bond that you guys have - I think that there's something really special about you two, and I'm definitely going to say yes."

Hailing from Gympie's AICM Rockskool program, the sisters won the junior section of the prestigious Aristocrats Entertainers talent competition in Tamworth in January.