Catherine of Siena (Caterina) was a twin whose twin sister, Giovanna, died around the age of two. It is argued that a conflict relating to her lasting relationship with her dead twin is the key to a psychological understanding of the mysticism of Catherine of Siena. She was torn between her survivor-guilt and her desire to be re-united with her lost twin. This 'Vergotean' thesis is supported by contemporary psychological knowledge relating to the social construction of twinship and the impact of the death of a twin on the surviving twin. This conflict was played out in her (twin) relationship to Jesus within the social context of her becoming an important religious and political leader of a 'family' of followers, including her biographer, the Dominican, Raymond of Capua.