Baroness Emmuska Orczy 
Heart Of A Woman 

Two young men are murdered, each in a carriage with a stiletto through the throat; one in Brussels, one in London. In the latter case the murder weapon, a stiletto stick, indubitably belongs to our young hero, Luke, and he is known to have quarreled with the victim in a club just before. In fact, Luke is protecting his elderly Uncle Radclyffe, a surrogate father to him, who has killed not once but twice (a real newly-discovered heir, and then a fraudster who impersonates the murdered man and blackmails the murderous uncle) in order to protect the inheritance of the only member of the younger generation of the family that Radclyffe cares for – Luke. (Goodreads)