HELP!

i read this story a while ago and i cant seem to find it, it was a story about a girl and her mum was in a relationship with Ian's brother and they moved somewhere (cant remember where) and she went and met Ian and they ended up kissing and she said it isnt right because she was only 15 but he insisted? please if you know who wrote it or what its called can you tell me, been looking for months :( THANK YOU