It's funny, I read through the Warbreaker chapters over the long weekend and was wondering if you (Brandon/EUOL) would find my comments useful, and one of my comments was going to be that I thought the sisters were a little too similar! Goes to show how different readers notice and focus on different things.
I'm not sure how valuable my comments will be, in that Warbreaker feels very much like a young adult novel and that's something that I don't typically go for these days: the genre post-Harry Potter has too many characteristics that I find unfortunate, although they're undeniably viable among many readers. However, realizing this is a first draft, here are some specific thoughts I had on Siri and Vivenna while reading the chapters, which you can use or not as you think best.
Regarding differences and similarities between Siri and Vivenna, I think there's a bit too much "tell" and not enough "show" with their characters and characteristics. Several different sources tell us repeatedly that Siri is impetuous and undisciplined while Vivenna is dutiful and thoughtful. It is Siri however who goes obediently off to marry the God King (as opposed to doing something impetuous, like running away), and keeps her discipline by thinking of her duty to her country; it is Vivenna who, impetuously and in defiance of her father, rushes off to save Siri with no real plan. Thus, what we're told about the characters and what their actions show us are consistently two different things. This is a problem in itself, of narrative, and also it makes the sisters feel too similar. Both are a similar mixture of impetuousness and duty; I can see both acting in nearly identical ways if their positions in the book were reversed (Vivenna going dutifully away to marry the God King, Siri setting off impetuously to save her from this fate, etc.).
Some of this I'm sure is intentional -- the "circumstances force two seemingly opposite characters to realize they have more in common than they thought" theme. My comment though is that not enough is done to show them as opposite in the earlier chapters, and they come too quickly and with too little transition to their similarity.
(A side note on the sisters and names: "Vivenna" sounds like "viv," i.e., vivacious, while "Siri" sounds like "serious" -- their names are almost the exact opposite of their stated characteristics.)
One problematic aspect of the sisters' similarity, from a storytelling point of view, is that both, having been thrust into unfamiliar situations, have been very passive for the past 15-20 chapters. An example that stands out for me is the scene where Vivenna gains Breath. Vivenna is asked to make a moral choice to accept or reject the Breath, she chooses to reject it...and then she gains it anyway. Perhaps this is meant as a statement on life, that sometimes burdens we didn't want are forced on us anyway, but it felt too much like a contrived plot device, a way of getting Breaths to Vivenna that the story will require her to have without requiring her to take action that would impact her moral and theological principles. Indeed, part of the problem I think is that she doesn't react as we might expect her to, given that she's portrayed as much more religiously knowledgeable and devout than Siri (again, show vs. tell). I'm thinking her reaction should have been that of an orthodox Hindu given a leather jacket as a birthday present, or a vegan who was force-fed a mouthful of veal: they'd want it out!off!away!, ASAP. Vivenna's lack of action in this regard pretty much announces her eventual acceptance of the Breath as a practical tool, and makes her subsequent expressions of disgust over the various Hallandren practices somewhat tedious to read.
In fact, as a general statement most of the main characters have seemed to me too passive after the first few chapters. Siri is swept along by court affairs, Vivenna is swept along by the mercenaries, Lightsong is swept along by his high priest, etc. I think that this is a big part of why people keep asking for more Vasher: he is the only character who is actively forging his own path and appears to have a plan of action.
Of course, that's just my $.02, and I realize this is a first draft that I haven't yet read the ending of -- some of my criticisms may resolve themselves in the story you already intend to tell. Indeed, I both have to thank you for making a work in progress like this available -- the process of creation fascinates me -- and say how impressed I am by the quality of the superstructure you're assembling in this draft. It's quite a complex web you're weaving, with all the different characters and mysteries, and wanting to learn how it all turns out is a powerful incentive to keep on reading!
Best,
MattD