The Evil Queen's beauty is external, and constantly called into focus by Ravenna herself (internal), while Snow White's beauty is of the "inner beauty" variety, and brought up by others (external).
The fact that the voice of the Mirror is male. Ravenna hates all men save her brother, but in the end, she still internally seeks approval from them by her imaginations of the Mirror. It's implied that it's all in her head rather than an actual magic mirror, which makes it all the more clear that she's damaged goods and more than a little insane.
Kristen Steward seems like a somewhat odd chioce to play the "Fairest of them All", at least against Charlize Theron. (she's nothing to scoff at, it's just the comparision.) But a) is it Snow White's inner beauty that's more important, which Ravenna doesn't see or care about. b) It's the mirror who tells her about Snow White being more beautiful than her. The mirror - unlike her other magicks maybe is not perceived by others, even when she's talking to it with her brother next to her (Finn is not mirrored in the Mirror Man... thing). So, Snow White being declared "The Fairest" is only in Ravenna's head. Why? Because she knows Snow White's the rightful heir and is now grown and old enough to challenge her. Finn probably told her about how stubborn she is... so she translates this fear into the only thing she's consciously afraid of: that she's not the most beatiftul woman (and thus, the one most likelyto come into power with her schemes) anymore. Her perception of Snow White as Fairest Of Them All has jack dung to do with Snow Whites actual looks, one way or another.