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

A youngish woman whose main occupation seems to be bird watching with her field glasses, Helena Sommers lives with her sister in a small cottage on the Haldane's estate. Her main contribution to the investigation is that she saw Mark Wilton walking on the path above Mallows land on the morning of the murder. In the last chapter, the author reveals that Helena Sommers and Maggie Sommers are one and the same person. Helena is the true killer, wreaking a horrible revenge on the wrong man for a wrongful death.