According to The Wrap, the film will find Blunt playing Beatrice "as she investigates the death of her young sister whose death is ruled a suicide by police. Convinced that her sister was murdered, Beatrice set out to uncover the strange events leading up to Tess' death, though she may pay a terrible price for the truth."
Although there is no word as to who will write or direct the feature, Blunt is a credible lead and she has dramatic chops that can make Sister a truly intriguing thriller. You have my attention.