Song of the Forever Rains by E.J. Mellow

In which three sisters born 2 years apart from the middle sister on the same day, wield magic with music: Arabessa, with her violin, Niya, with dance and Lark, with voice.  The story opens with Lark, the youngest at four years, having never uttered a sound since her birth, when her mother died, are exploring a part of a castle where they should not be.  An immensely powerful guardian creature traps them in a dead-end while an all-powerful creature, Achak, who has two souls trapped in a single body, brother and sister, looks on.  At the last second, Lark turns around, looks the approaching monster in the eye and screams NO at the top of her lungs, shattering the creature.

Achak takes over the teaching of the children, helping them learn how to control their magic, and how to use swords, knives, and other deadly arts so they don’t have to always use their magic.

We skip forward in time to Lark, at almost 19, disguised as a street urchin, having stolen a ring and getting her finger cut off.  She refuses to let any sound out, because that would kill everyone around and make it obvious that she has magic.  She meets a well-dressed man who seems to be lost and leads him to safety, but not until having to use a knife to defeat a group of thugs with swords.  Once back to the part of the city that she is really from, she gets cleaned-up and is presented to her father.  He tells her she is to be pseudo-engaged to a Duke so they can find out what is going wrong in his part of the kingdom and steal his treasure.  Her father is also the King of Thieves.