The concept of moving up- and down-stream in realities was last read by me in Hal Duncan's "Vellum", and I really didn't like that book. However, I love the concept itself, and if that's what the book's going to be about, I'll probably give it a chance.
As for the story -- I liked it, and while I'm not 100% sure Dariael was doing the right thing by killing her younger sister (the MC) before the MC could do all this evil she said she was going to do, it certainly is an interpretation that gives the story that much more gravitas. It explores the concept of "doing evil for a greater good", whereas the father, by saving the MC, is "doing good that causes evil later" -- kind of like the person who taught Hitler to swim at age five so that when he fell in a lake at age twelve he could swim out*.
* Just an example. I'm sure it didn't really happen. If you prefer, replace "Hitler" with "Pol Pot", "Mussolini", or whatever American politician you hate most.
Good reading, too.