I read it at middle school for 15 or so years ago.
There was a brother and sister. English mum, Greek dad. The dad picked them up from school one day, abducts them and takes them to Greece. The boy sends letters to his mum but the post person holds them back.
Big court battle, little girl doesn't remember mum and stays with dad. Boy goes home to mum.
I loved the book, it's stayed with me for years but I can't find out what it was called.