A Whisper in the Walls by Scott Reintgen

In this sequel to the bestselling, “pulse-pounding” (Publishers Weekly) A Door in the Dark, Ren’s intellect and cunning are stretched to the limit in her quest to take down the system that stole her father’s life.

Ren Monroe is a wolf among lions. After infiltrating one of the greatest Houses in Kathor through her successful bond with Theo Brood, she finds Theo’s father is two steps ahead. He exiles Theo and isolates Ren, strategically working to break her unwelcome grasp on his son—and foiling Ren’s first step to enacting the revenge she’s been planning her whole life.
Ren might have more resources than she’d ever imagined growing up, but she’ll still get nowhere without allies. Enter House Tin’Vori. Years ago, House Brood led an unprecedented raid to destroy a fellow House of Kathor. But a few siblings survived, and they haven’t forgotten the horrors waged against their family. Quietly, they’ve plotted their own revenge, waiting for the right moment to strike. And Ren Monroe might be their best chance.
Like fire, the Tin’Vori siblings are as dangerous as they are useful, both gifted in rare magics. Ren must decide how to unleash them against House Brood without hurting Theo in the process. Her feelings for Theo are growing past the boundaries of their bond, and Ren finds herself balanced on a knife’s edge, a breath away from immense power or utter ruin.