For specific context, the question arose out of discussing Han Solo calling Princess Leia "sister" in "Star Wars" Episode IV.
What is the etymology and context of using the term "sister" in this way? (unrelated woman that you're projecting a sense of familiarity onto).
Is it from the US Southern culture? From earlier decades of 20th century? Else?
Best Answer
Christian culture regards all of humanity (or, in the more up-tight versions of the religion, all Christians) to be brothers and sisters. (This does not imply a literal common parent, but rather a spiritual kinship.)
The use of "sister" to refer to an unrelated woman is certainly not practiced extensively in most of the (white) US, but the term is used in that sense in some religious sects, particularly in the US South. And, while I have little first-hand knowledge (and Urban Dictionary is useless here), I've been led to believe that the use is common in certain African-American subcultures.
Using the terminology in Star Wars, in the fashion it was used, would tend to re-enforce the sense that the speakers are members of a common cult or culture -- I don't think there's any deep hidden significance.