Hi! Does anyone know the title of a mystery involving an American woman seeking to solve the mystery of her twin sister's death, and part of the plot has to do with orchids - this is set in France.  I know it's a long shot, but maybe someone out there knows what I'm looking for. 

Is it Deadly Slipper: A Novel of Death in the Dordogne (2005) by Michelle Wan?

Wan has two other books in the Mara Dunn series: The Orchid Shroud (2006) and the newly released A Twist of Orchids.

Here's a description of the storyline:

Nearly twenty years have passed since Mara Dunn’s sister Bedie, an orchid enthusiast, disappeared while on a hiking holiday in southwestern France. Mara remains determined to find out what happened—but her only real clue is a sequence of wild orchids, including a mysterious, previously unknown Lady’s Slipper, captured on a roll of film found in Bedie’s long-lost camera. With the help of Julian Wood, a reclusive English botanist, Mara begins her search . . . stumbling into decades’ worth of local secrets and putting herself in danger. Rich in lush descriptions of the Dordogne, and laden with savory details of French cooking, Deadly Slipper is rife with surprising twists and turns.

Yes!! That's the one.  Thanks so much for helping me to find it again.  I'll read the other two you mention for sure. 

Vous êtes bienvenu. (I only took one year of French in college--hope this is the correct response. LOL)