She was portrayed by Janette Scott.
For she lived in Femm Hall her whole life so she could have her claim to the share of the family fortune (a long standing will stated, only the last member that stayed in tha house could have it), at first seemingly normal unlike her family (each one caught up in an obsession or unsual behaviour for them to deal with the will's specifications), but her whole life she lived in fear of the house and what it did to her family. After many years this finally drove her mad.
Now consumed with greed and insanity, she started murdering her family so she could have the fortune and be free of the house. She killed her cousins, Casper and Jasper (Peter Bull), her aunt, Agatha, and her uncle Roderick (Robert Morley) and tried to kill the rest of her family and destroy Femm Hal l itself to be finally free of it. But her plans it hit a bump when Tom Penderal (Tom Poston) a buissness assosaite of Casper, who had came to the house and got involved with her dark plan. Although originally seeming sweet towards him, as Tom got deeper and watched the murders. At the climax he managed to figure out her plan and her style, revealing it to be true Cecily revealed her true insanity and held her sister at knife point planning to kill her, she offered Tom the chance to join her and live through this with all the money. But realising how insane she truely was Tom turned her down, furious she hurled the knife at him and imprisoned her sister inside the house, leaving them both to die. However figuring out her style (she had killed intime with the many clocks inside the house chimming and with the old nursery Rhyme "Oranges and Lemons") Tom thus managed to find and disable all the pieces of dynamite she had planted in time, by chance he accidentally threw the very last active piece to where Cecily was standing right before it exploded killing her.