Down Among the Sticks and Bones by Seanan McGuire

Down Among the Sticks and Bones
A dark fairy tale about two sisters, Jack and Jill, who, when they're children, find their way into a sinister world ruled by a magnetic man called The Master and a weary scientist. Jack is the beauty and Jill is the tomboy and both girls transform into what they want to be in a land on moors surrounded by seas and forests containing terrifying beings. The sisters' rivalry becomes a breathtaking and poignant journey as they mature in this world. It's a beautiful and eerie tale of sisterhood with touches of horror. A Wrinkle in Time for anyone who loves Neil Gaiman and Caitlin Kiernan.