"Perhaps two young brothers' hip-hop moves complement big brother's rap lyrics.
"Maybe Mum and her two daughters carry the harmony as Dad and his two sons carry the backline of drums and guitars."
The programme makers are also after families with interesting background stories, like Gareth's famous stammer on Pop Idol.
The spiky-haired star overcame his problem to hit the top of the charts, making his rise to fame even more exciting.
Some of the biggest groups in pop had members from one family, like The Jackson 5, The Osmonds and The Bee Gees.
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