The Cloak and Dagger #1: Anvil of the sun by Anne Lesley Groell

Aristocratic assassin Vera Devereaux takes on a routine job of killing a politician, and, counter to custom, is promptly arrested for the murder as part of an ingenious scheme. Her rebellious debutante niece, who takes after Vera in many ways, rushes to rescue her and becomes enmeshed by the local revolutionary movement and its charismatic leader. This is the first of a series, but the story is complete. Ignore the said-isms and shaky social premises and enjoy the fast-moving plot. Groell has a wicked eye for farcical timing. If she tightens her control of language, she will give us some very funny fantasy of manners modelled on this one.