If your sister is a Sister in the Roman Catholic Church, then she is a votary. People who have devoted their lives to religious service are called votaries.

The noun votary does not necessarily have to be a member of a religious order. A votary can be a person who is a devoted follower of a person or cause. Plenty of history majors are votaries of Abraham Lincoln, but if you prefer more obscurity in your interest in presidents, you may be a votary of William Henry Harrison, who died on his thirty-second day in office — of a cold.

Definitions of votary
  1. noun
    a priest or priestess (or consecrated worshipper) in a non-Christian religion or cult
    “a votary of Aphrodite”
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    vestal virgin
    (Roman mythology) one of the virgin priestesses consecrated to the Roman goddess Vesta and to maintaining the sacred fire in her temple
    (classical mythology) a priest or votary of Bacchus
    (classical mythology) a priestess or votary of Bacchus
    Rea Silvia, Rhea Silvia
    (Roman mythology) a vestal virgin who became the mother by Mars of the twins Romulus and Remus
    type of:
    non-Christian priest, priest
    a person who performs religious duties and ceremonies in a non-Christian religion
  2. noun
    one bound by vows to a religion or life of worship or service
    “monasteries of votaries
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    type of:
    a member of a religious order who is bound by vows of poverty and chastity and obedience
  3. noun
    a devoted (almost religiously so) adherent of a cause or person or activity
    “the cultured votary of science”
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    type of:
    adherent, disciple
    someone who believes and helps to spread the doctrine of another
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