The Stranger at the Door — R.E. Loten

The second book in the Isle of Legends series takes you on an adventure around the British Isles, on the trail of Norse legends. Freja and Lukas are twins with a history. They have known all their lives that they are adopted, but have no idea who their real parents were, until the strangers arrive at the door of their family home to tell them that their elder brother has been injured in an accident at his camp in Afghanistan. From that moment on, the twins are thrown into a series of events that reveal their true heritage. Following on from 'The Forest Children' this can be read as a standalone book, or as part of the series.