The real name of Bavarian princess and Austrian queen Elizabeth is Elisabeth Amalia Eugenia von Wittelsbach, nicknamed Sisi. Judging from her signature, it should be Sisi (although some people infer from her handwriting that it is actually Lisi who is more likely). Cici seems to have started from the film Sisi Trilogy, and has also appeared in film and television literature since then.
Elizabeth herself has four kinds of signatures: Princess Cece, Elizabeth, Erz Sibbett and Tania. Erzsebet is a Hungarian name, and Titania is the pseudonym used in her poems, which comes from Shakespeare's A Midsummer Night's Dream.