I _loved_ the two earlier books in this series, and I loved Kritzer's other books. But this one stumbles. I'm sure that, if you read Freedom's Apprentice and Freedom's Gate, you'll buy Freedom's Sisters too -- just because you need to wrap up the "...and then what happened?" the author left us with; however, you may end up saying, "oh" instead of "wow."
Kritzer does tell us more about the world in which her characters live: where Alexander the Great didn't die young, where slavery is rampant, and where djinn were captured to perform magic for their owners. (And darned good worldbuilding it is, too.) In the final book of the trilogy, she fills in more of the political landscape, but it's mostly painted in the background or reported by a secondary character. Yes, you do find out about what it means for Lauria to be a "gate" and the promise of the rivers' release. But by that point, I'm not sure that I cared to know as desperately as I once did.
One reason is that the two main characters, Lauria and Tamar, are separated for most of the book. Kritzer has to switch back-and-forth between each character to move the action along, which in this case made the story flow uneven. Kritzer's also so busy trying to move the characters through the story that she does less painting of the background; in earlier books, we get the taste of kumiss, the smell of the tents, the heat of the day -- our senses are engaged. Not so much, here.
A larger problem is that Kritzer's characterizations aren't up to her usual excellent quality; people change their minds or make decisions for apparently random reasons, and their motivations are unclear. (I'd give examples but they might become spoilers.) Is Kyros a good guy or a bad guy? What's going on in his head? She could have accomplished a great deal by giving *his* viewpoint in the story.
I did read the book all the way through, and I was glad to find out what happened to the Alashi. Despite my criticisms, I suspect that most readers of #1 and #2 will feel compelled to read #3. It's just that Kritzer is ordinarily such an awesome writer that this single stumble is a disappointment -- like an A+ student unaccountably handing in a B- term paper.