When she was 13, her father took her on a fox hunt, and she enjoyed the blood and gore. Charley said that this changed her. (AUDIO: The Next Life) Cecelia was a member of the League of English Fascists and had met Adolf Hitler. (AUDIO: A Blind Eye)
Torvald, an agent of the Celestial Intervention Agency, tried to threaten Romana's position as the Lady President of Gallifrey. First, he brought the anti-time crisis to light. Next, he changed history slightly. He lured Sissy to Switzerland with a fake telegram from her long-missing sister, Charley. Without that interference, Sissy would have committed suicide in Munich to protest the beginning of World War II.
On 1 September 1939, a meeting with Leela made her realise that her blind eye to the Nazis' crimes made her just as liable as them. Sissy shot herself in this new timeline after writing a farewell letter to Charley. (AUDIO: A Blind Eye)