Mumsnet has never yet let me down when I've wanted a book found with very little information to go on!
This time it's a tete beche book (one where you read half the book, then turn it upside down and flip it over and start again from the back, which is now the front). It was a YA book, and I would have read it between 2001-2004. Each half was from the point of view of a different sister (both teenagers). The younger one was unhappy and unpopular and jealous of her older sister, who she described as being popular and self-obsessed- one bit I remember was her talking about how she saw her sister being cat-called by builders and how she was obviously loving the attention.
Then when you read it from the older sister's POV, you realised that actually the younger sister was devious and manipulative. And you found out that she had hated being cat called, but she smiled and played along because she was scared of how the men would react if she didn't.
That's all I can remember. If anyone can track this book down for me, I'd be thrilled!