Jim Hines' The Stepsister Scheme and The Mermaid's Madness

Also, Steven Brust. The man couldn't write a flat character if he tried.

Not fantasy, but Scott Westerfeld's Uglies trilogy has one of the best/most complex female relationship I've ever read. (Don't be put off by the covers, the books are amazing. In a just world, there would be just as much femslash between the two female characters as there is between Harry and Draco, and with more reason.)