Sister/Origin

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Origin 1
When Sister was pretending to be human, she was disguised as a nun. A nun is a member of a religious community of women, typically one living under vows of poverty, chastity, and obedience. She may be a woman who decided to dedicate her life to serving all other living beings, or to be an ascetic who voluntarily chooses to leave mainstream society and live her life in prayer and contemplation in a monastery or convent. The term "nun" is applicable to Catholics (eastern and western traditions), Orthodox Christians, Anglicans, Lutherans, Jains, Buddhists, Taoists, Hindus and some other religious traditions. While in common usage the terms "nun" and "sister" are often used interchangeably (the same title of "Sister" for an individual member of both forms), they are considered different ways of life, with a "nun" being a religious woman who lives a contemplative and cloistered life of meditation and prayer for the salvation of others, while a "religious sister", in religious institutes like Mother Teresa's Missionaries of Charity, lives an active vocation of both prayer and service, often to the needy, sick, poor, and uneducated.

Origin 2
Emperor Sutoku reveals Sister to be a demi-goddes, do it is unsure who she is exactly. A demigod (or demi-god) is a divine or supernatural being in classical mythology. The term has been used in various ways at different times and can refer to a figure who has attained divine status after death, a minor deity, or a mortal who is the offspring of a god and a human.