Sister of Starlit Seas by Terry Brooks

A rebellious young heroine begins a voyage of self-discovery in the third novel of an epic fantasy series set in the world of Viridian Deep, from the legendary author of the Shannara saga.
Auris's adoptive sister Char has always been the baby of the family-a position that grates on Char, especially when everyone insists on telling her exactly what to do and how to do it. But Char is certain that her headstrong, impulsive behavior, the quality her family sees as her greatest weakness, is actually her greatest strength: the willingness to instantly brave danger and leap to the rescue when anyone she loves is threatened.
Char knows she will never grow into the woman she was meant to be under her family's loving but repressive eye, so a month before she turns fifteen, she runs away and joins a Human pirate crew in the warm southerly regions of her world. Then, three years into her pirate career, her captain-the man she is convinced she loves-is captured by the leaders of the slave trade he has been fighting. When Char leaps in to rescue him, she finds herself thrust into an adventure that will uncover secrets she never suspected about herself, one that will maybe, finally, teach her to look before she leaps.