The setting was probably the biggest weakness -- it had too much of a 'generic fantasy' feel to it for my tastes. Like it was there to meet the readers' expectations of "fairy tale fantasy setting written in modern style" without a lot of thought put into "why it makes sense for the world to be this way". Not in all respects -- the treaty between humans and fairy folk was a nice piece of plot-critical backstory, for instance. But enough so that I found myself having a hard time suspending disbelief and accepting the setting.
Lut commented that the cover looked like "Charlie's Angels Fantasy Series", and it felt rather like that, too, which didn't really help. Two of the book's three princesses are from other kingdoms and secret agents for the queen, posing as servants for unclear reasons (to be inconspicuous, I guess?) with special powers -- Snow White is a sorceress with general mirror-based magical powers, Talia aka Sleeping Beauty is a combat monster. Danielle aka Cinderella can persuade animals to help her, rather like the Disney incarnation. As superpowers go, Danielle's is the most understated, and the best utilized. But the princesses-as-superpowered-secret-agents did strike me as contrived. One of the reasons I liked Danielle best is that (a) she didn't seem as powered-up as the other two and (b) her motive was 'save husband' instead of 'I'm a secret superagent'.
Things I did like: Danielle and Snow were both entertaining, likable characters. The villains were hateable and I hated them. At a number of points in the book, Danielle comes up with clever solutions to difficult problems, and I always admire that in a character (and an author). The story is reasonably well-paced and moves along well. The characters have clearly laid out competencies and weaknesses and the author sticks to them.
Overall, it was not a bad book; I'm giving it a 6. If someone wants to stick up for the sequel, The Mermaid's Madness, as being an improvement upon my issues with The Stepsister Scheme, I'd be willing to give it a shot at some point. :)