"The Killing Song" by Felix Graves

Romada, a fiercely independent water nymph who captains the pirate vessel the Guardian, is looking forward to retirement. After sailing the seas for over 50 years with her siren girlfriend, Neiara, as well as with her adoptive big brother, Injago, for the five decades prior, she's tired of the high-stakes life. Unfortunately, the high-stakes life isn't done with her or her loved ones yet.
 

When the crown prince of Ostrana is assassinated by a siren, the elven kingdom responds by rounding up every known siren and shipping them off to internment camps. Soon after, Injago and Romada mistakenly lead Neiara into a trap that tears her away from them, and Romada succumbs to a depressive darkness that has threatened her mind since childhood. It's up to Injago to keep the crew and the ship safe and steady on the path to finding and rescuing Neiara.
 

As time goes on, Romada and Injago realize that no one else is willing to find and liberate these camps. They have a choice to make. They can attempt to free all sirens of their wrongful imprisonment, or abandon these lost souls in reckless pursuit of their stolen family member. Only one thing is certain: this won't have a happy ending.

"The Killing Song" is part of the Stories of Faerth by Felix Graves.