Habit of Despair — Robert Barclay

This romance is cast in the turbulent times of Ferdinand and Isabella in the early years of their reign. As the two young monarchs struggle to curb the nobility, reform the church, and reconcile the minorities, the king and queen are beset with special interests, rampant corruption, and political forces that resist the changes that will soon make Spain the premier power in Europe.
In the context of this changing world, Ricardo DQuintillion and his sister, Maria, struggle to support their monarchs while dealing with the passions and intrigue that beset their lives and threaten to bring ruin down upon them.
Ricardo, newly appointed to lead the Hermandad, a top civil-military post, is given responsibility for maintaining law and order and bringing the lawless aristocracy under control.
Maria is left to run the estate but soon finds herself masquerading to find her brothers estranged fiance. She stumbles onto a secret religious order seeking to establish the Inquisition.
As the siblings struggle to serve, their strong passions create stumbling blocks that impede them from accomplishing their goals.