Pink Chameleon by Fiona Dunbar

Rory and Elsie's creative parents have gone missing, and they have now been put under the guardianship if their awful Uncle and Aunt. They also have to live at the boring, strict old boarding school Poker Beaut Hall. Bu the sisters are determined to escape from their uncle and find out the truth behind their parents disappearance!

I recommend this book for girls aged 9+.