The Trylle Trilogy by Amanda Hocking

When Wendy was 6 years old, her mother tried to kill her. She was convinced that Wendy was not her daughter, but a monster who took the place of the son she was supposed to have. Wendy ended up with her aunt and her brother Matt, who stopped their mother from murdering her. They are constantly on the move because Wendy never seems to fit in anywhere. After their newest move, Wendy notices Finn, a cute guy in many of her classes, who is constantly watching her but never speaks to her. When he finally does, he reveals the truth to Wendy. At first, she thinks he's crazy, but as events unfold, she has no choice but to accept who she really is. 

I really enjoyed this series. The characters are very well-developed (Loki is my favorite!) and the storyline kept me guessing. I'll admit that it's hard to say much about it without giving things away, so my suggestion is to just go and read it!