"Angel of Vengeance" by Preston & Child (Agent Prendergast series, Volume 22)

A desperate bargain is broken…

Constance Greene confronts Manhattan’s most dangerous serial killer, Enoch Leng, bartering for her sister's life – but she is betrayed and turned away empty-handed, incandescent with rage.
 
A clever trap is set…

Unknown to Leng, Pendergast’s brother, Diogenes, appears unexpectedly, offering to help—for mysterious reasons of his own. Disguised as a cleric, Diogenes establishes himself in New York's notorious Five Points slum, manipulating events like a chess master, watching Leng’s every move…and awaiting his own chance to strike.
 
A vengeful angel will not be deterred…

Meanwhile, as Pendergast focuses on saving the unstable Constance in her fanatical quest for vengeance, she strikes out on her own: to rescue her beloved siblings from a tragic fate and take savage retribution on Leng. But Leng is one step ahead and has a surprise for them all…