Lia and Alice Milthorpe have just lost their father and soon after his death, a strange circular scar appears on Lia's wrist. It marks her as a part of the prophecy, although she doesn't precisely know her role. Is she the Guardian? Or is she the Gate? Truth be told, she doesn't figure that out for a while and it's James--the boy that she's known for quite some time and fell in love with--who finds the book that contains a single page of the prophecy. Throughout Prophecy of the Sisters Lia learns about well, the prophecy and her role in it; she also witnesses a scary change within her sister who plays for the opposing side of the prophecy and learns about the Keys which can either help release Samael or destroy him--although ultimately the decision rests with the Angel of Chaos. 
 Unlike Alice, Lia begins with no knowledge of the prophecy or her powers. Through a series of events, she stumbles across the prophecy with James' help and from there she meets several minor characters who help her learn bits and pieces of the prophecy. Meanwhile, Alice tries to sway Lia to join her cause and Lia--rather obviously--resists. 
 There's more that I could add to my summary, but I won't, because they're spoilers. But I will say that I loved Michelle Zink's writing style and that it flows really well in my opinion (beautiful and lyrical). Alice is frightening. No, really, she's pretty scary and I can't believe her level of crazy--although one could blame the Souls. Lia on the other hand it pretty oblivious when the story begins and slowly, she learns to see things for what they really are (figuratively). And then there's James--who Lia has known for quite a while and happens to be in love with. If it wasn't for him, Lia would've never even heard of the prophecy and although she tells him nothing of the prophecy, he does pop up often enough--as he was cataloging her father's library. 
 Prophecy of the Sisters is set in 1890 and a bit north of New York City--which automatically meant that my interest was piqued. It features two twin sisters set on opposite sides of an age old prophecy and did I mention that the cover is gorgeous? Admittedly this has been compared to Libba Bray's Gemma Doyle series and well, the only thing that I feel that they have in common is magic and the fact that they're both set in the Victorian era. This is a book that I'd recommend and although perhaps not everyone will love it, I personally feel that this is definitely a book to try.