Melinda Griswald 
The Scarlett Ruby [EPUB ebook] 

Scarlett and Nina are twin sisters growing up in a small town in Illinois. Their perfect little life begins to crumble when their parents separate, and things go downhill from there as Scarlett starts exhibiting strange and bizarre behavior that puzzles Nina and makes her fear for her safety. As they get older, Scarlett begins to lose her grip on reality, and it»s up to Nina to help her sister before she hurts someone — or worse, hurts herself.