Though it begins as a Merchant-Ivory exercise, one of those slow-moving explorations of class dis-tinctions in a wealthy English household during an earlier time, "Sister My Sister" takes some twists that veer off into quite unexpected directions.

The first revelation is that, despite British accents everywhere, the characters are actually French. And what initially seems merely "slow-moving" rather quickly becomes "dull" and "plodding." This despite the development of a lesbian-incest relationship that ultimately takes a homicidal turn.

Based on a true incident that occurred in the 1930s, "Sister My Sister" begins with a black-and-white prologue that shows two deeply attached sisters, Christine and Lea, being separated during their youth.

Eventually, we learn that their mother has kept them separated, sending Christine first to a convent and then out into the world as a maid and cook for wealthy families. All the money she earns goes back to her mother, of course.

But now, after settling in with crusty old Madame Danzard (Julie Walters) and her dour daughter Isabelle (Sophie Thursfield), Christine (Joely Richardson) sends for her younger sister Lea (Jodhi May), offering to Madame Danzard the services of two for the price of one — and they'll even share the same bedroom. How can Madame resist?

Though she is ecstatic to return to the company of her sister, self-effacing Lea is quite nervous about the work, and her integration into domestic service is slow and painful. She takes solace in the comfort of Christine and it isn't long before their relationship takes a sexual turn, which apparently it had also done in their youth.

Before long, it becomes apparent that Christine is rather unhinged, and as she gradually moves away from whatever sanity is left, she takes Lea with her.

The performances here are all quite good and the story is unquestionably fascinating. But the filmmakers (director Nancy Meckler and screenwriter Wendy Kessel-man) allow the proceedings to become unmercifully boring (and some of the sex scenes go on too long and are gratuitously specific).

"Sister My Sister" is rated R for violence, incest, lesbian sex and partial nudity.