Who is Helena Sommers from A Test of Wills and what is their importance?

Helena Sommers is a young woman with a penchant for bird watching. She lives with her sister Maggie in a small cottage. On the morning of the murder, Helena claims to have seen Mark Wilton on the path, something that makes him the primary suspect in the murder. In the end, we learn that Helena and Maggie are actually the same person, and that it is Helena, not Mark, who did the killing.