From the experts:
Sister is an old-timey nickname-name for girls, ranking in the Top 1000 as a proper name for girls until the beginning of the 20th century. But more often, Sister was used as a nickname in the truest sense of the word, not a short form ala Kathy but a nickname in the way that Chip and Bud are. Or maybe Junior is a more appropriate name analogy: Sister was sometimes the nickname given to the only girl in a family of boys, so literally a descriptive word name like Junior. In the most recent year counted, Sister was given as a full name to zero baby girls in the US and was probably used in a casual way by not that many more.