Cheney of Faire Isle Trilogy by Susan Carroll
The Cheney sisters trilogy--The Dark Queen, The Courtesan, The Silver Rose--follows three "daughters of the Earth" who live during the time of Catherine de Medici. The books are a mix of historical fiction and romance with a touch of the paranormal.
The Dark Queen is the story of oldest sister Ariane, who inherited the title of "Lady of Faire Isle" along with a cache of priceless magical and medical texts are the ire of the Dark Queen Catherine de Medici from her mother.
In The Courtesan, middle sister Gabrielle becomes a famous Parisian courtesan and must decide whether she can risk nearly everything to save the one man she loves.
The series concludes with The Silver Rose, in which the youngest sister Miribelle helps vanquish the great evil that is known as the Silver Rose.
The Cheneys are well-drawn and sympathetic, as are their heroes. The books are engaging, enthralling, and have enough seriousness and historical detail to make them more historical fiction than historical romance (in my opinion).