Sweet Nightmares. The Dream World Chronicles, #1

She's never had a nightmare in her life. She doesn't know that's what makes her a warrior. The one thing fifteen-year-old Alex Rokosz has endlessly craved is a family. So when her mother's death forces her and her sister to move in with Matilda, the grandmother they never knew they had, Alex hopes to bond with her and find out what happened to her missing father. Instead she encounters numerous oddities at her new house, including Kacper, her grandmother's suspicious seventeen-year-old housemate.
But things get even stranger when Alex catches Matilda and Kacper slaying a real-life nightmare. Overnight, she is swept into a world where sleeping is dangerous, nightmares are real, and imagination is magic. She discovers there's more to her family than she realized - and her own destiny is far greater than she ever imagined.