Sister by Rosamund Lupton

Their bond was unbreakable.
The truth was unimaginable.

When Beatrice gets a frantic call in the middle of Sunday lunch to say that her younger sister, Tess, is missing, she boards the first flight home to London. But as she learns about the circumstances surrounding her Tess's disappearance, she is stunned to discover how little she actually knows of her sister's life - and unprepared for the terrifying truths she must now face.

The police, Beatrice's fiancé and even her mother accept they have lost Tess, but Beatrice refuses to give up on her. So she embarks on a dangerous journey to discover the truth, no matter the cost.

Grabbed from the beginning and kept me interested.  Both the mystery of whodunnit and why and the way the characters changed and grew.   A really good twist at the end that I saw hints of but didn't fully see coming. An excellent book.