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.