The Pale Assassin by Patricia Elliott
Fourteen-year-old Eugenie de Boncoeur is a spoiled aristocrat living i the midst of the French Revolution. Her brother, Armand, is keeping many secrets from her. This annoys her very much, for she would like to know just what is happening in her country, but there is an even greater secret than the Revolution ideas that Armand is keeping.
Their guardian arranged for Eugenie to marry a wealthy man called Le Fantome. The arrangement was made for wealth, but Le Fantome hold a secret grudge against the de Boncoeur family. Can Armand keep his sister safe from the Revolution and Le Fantome?
Next Book: The Traitor's Smile