The Good Sister

Struggling with marital problems, a man begins a passionate affair with his wife's long-lost, twin sister.
The Good Sister follows a couple who is working to put their crumbling marriage back together, when the wife receives a letter informing her that she has a long-lost twin sister. First thinking that the new family member will bring the couple closer together, the husband then begins an affair with the sister and discovers that she is treacherous and dangerous.