Silver in the Bone by Alexandra Bracken

Born without a trace of magic, Tamsin Lark is no match for the sorceresses and Hollowers who populate the magical underground of Boston. But when the only parent she’s ever known disappears without so much as a goodbye, she has no choice but to join in their cutthroat pursuit of enchanted relics to keep herself—and her brother, Cabell—alive.
Ten years later, rumors are swirling that her guardian found a powerful ring from Arthurian legend just before he vanished. A run-in with her rival Emrys ignites Tamsin’s hope that the ring could free Cabell from a curse that threatens both of them. But they aren’t the only ones who covet the ring.
As word spreads, greedy Hollowers start circling, and many would kill to have it for themselves. While Emrys is the last person Tamsin would choose to partner with, she needs all the help she can get to edge out her competitors in the race for the ring. Together, they dive headfirst into a vipers’ nest of dark magic, exposing a deadly secret with the power to awaken ghosts of the past and shatter her last hope of saving her brother. . . .