It is reaalllly frustrating me. I remember almost all the story and lots of detail but not the title or the author!!! 
It was a book about a girl called Alison and her little brother Chris. Their parents are divorced and they live with her mother, in a flat I think. Her mum has a nervous breakdown and they are sent to live with their dad, who has remarried to a pretty young woman (Briony?) who has a daughter (Rosa?) of around Alison's age.
Alison's little brother adapts well to the new household but Alison senses the class distinction between her, her little brother, her mum and then her dad and his new family. She feels that if she adapts to the new house, who eat muesli and have a generally pleasant upmarket lifestyle that she is betraying her mum and her roots. She is also jealous of her step-mum and step-sister and does a few mean acts which she later feels sorry for.
In the end she starts to see the benefits/advantages of their new living arrangements, and settles in better to the house and her new school. She also develops a better relationship with her step-mum.
I would love to read the book again. Can anyone help with the book title/author? Thanks v much!