Lizzie's Leaving by Joan Lingard
Published by Puffin

Lizzie, aged 15, decides to leave the chaotic home and family she has always known, to join her own father, half sister, and step mother.   At first she is delighted with their luxurious home and lifestyle.   Gradually Lizzie realises that she is expected to care totally for her new sister, even when this conflicts with her own social life.   She forms a good relationship with her Dad, but does not feel accepted by her stepmother.   Lizzie is close to her sister Alice, and this relationship contributes to the dramatic events which lead to Lizzie returning home again.

This book explores relationships with step families as well as boyfriends in a realistic and entertaining way.