The Lesser Devil (The Sun Eater #1.5)

Crispin Marlowe grew up in his brother’s shadow, but Hadrian is gone, and his grandmother—the duchess of Delos—is dying. Answering her summons, Crispin takes off on what should be a routine flight to the ducal palace from the castle at Devil’s Rest.

But he is flying into a trap. There are whispers of treachery among the duchess’s many daughters, rumors of hidden knives in the planet’s court... if he can even make it there.

With no one coming to save him and only his sister, Sabine, and a few injured guardsmen to protect him, House Marlowe’s least famous son must make his heroic stand, and contend not only the foes arrayed against his house, but with the cultists who dwell in the mountains above his family’s ancestral home...