Autumn Storm (#2, Witchling Series). The Witchlings Series, #2

A girl with no memory, hidden in plain sight .A boy with no hope, spiraling into Darkness .Only one knows the truth, and only one has the power to save them. Autumn doesn't know why she remembers a boarding school she's never visited, or why her reflection is sometimes that of a ghost named Summer. She struggles to recover her memories and understand the warnings conveyed through her magick. Caught between the Turner twins, she isn't certain who to trust: Decker, the mysterious Master of Dark, who makes her heart catch, or Beck, the cautious Master of Light, whose touch soothes her pain.
Unable to recover from Summer's death, Decker slowly surrenders to the Dark. The guilt he feels when he falls for Autumn - and betrays Summer's memory - drives him farther from the Light, until he completely loses control and takes the souls of Light witchlings. As the protector of Light, Beck is caught between his allegiance to his twin and his vow to protect new witchlings. He must keep the Darkness from consuming his brother, for if Decker fails, he'll take down the only person who has a chance of saving them all.