Clarence and Emmeline Mumford are a middle-class couple, living in suburban Sutton on the outskirts of London with their two-year-old son. Reading the newspaper, they become aware of a young woman seeking a place as a "paying guest", or lodger. To supplement their income they respond to the advertisement and meet the prospective tenant, named Louise Derrick, who is in need of a place to live due to disagreements with her immediate family. Louise, who is poorly educated and bad-tempered, is being romantically pursued by two men: Mr Bowling, who is courting Louise's stepsister Cecily, and Tom Cobb.
The Mumfords do not get on well with their "paying guest", and attempt to persuade her to leave. This does not happen, and a series of events further disrupts the Mumfords' lives, including a private meeting between Louise and Clarence Mumford, which makes Clarence's wife jealous. Louise briefly and half heartedly seems to encourage Bowling's wooing of her, now that his courtship of Cecily is over, but Cobb makes a surprise trip to the Mumfords' home to pursue Louise. Louise, tripping on a chair Cobb had knocked over during the confrontation, accidentally starts a fire in the drawing room of the property, injuring Louise, who is then confined to bed for several weeks.
Eventually, Louise's time as the Mumfords' "paying guest" comes to an end, and she marries Cobb.

Question: Who is Cecily to Louise Derrick?
Answer: Stepsister 
Alternative answer: Step sister