Tin Grin by Catherine Robinson

Mattie’s glad for her mum when she meets a new, kind (albeit a bit scruffy) man. But then they drop the bombshell-her mum and Geoffrey are getting married! Not only might Mattie have to wear a flowery bridesmaid’s dress from hell, but Geoffery’s son Rupert is coming to live with them. It might be just about be bearable if Rupert was an adorable little five-year-old, but he’s a pre-pubescent, spotty, computer geek. And she’s sure he’s got it in for her-so much so that he makes a complete fool of her in front of the most gorgeous boy in school: Sam Barker. Mattie devises a cunning plan to get revenge… but nothing could have prepared her for the disastrous consequences. Consequences that force her to confront the horrible truth.