I've been a soul sister for more than 60 years and I've always assumed that "soul sister" only meant "a Black female", just as "soul music", and "soul food" only referred to Black culture (or specifically to African American culture).
I know that those phrases weren't coined until the 1960s. But I lived with them so long, that I assumed that the forms of the words might be updated, but their meanings would never change.
"Soul music" is probably the earliest of the "soul = Black folks and/or Black culture" references. Here's some information about how the word "soul" came to refer to Black (African American) music:
"According to the Rock and Roll Hall of Fame, soul is "music that arose out of the black experience in America through the transmutation of gospel and rhythm & blues into a form of funky, secular testifying...The term "soul music" itself, to describe gospel-style music with secular lyrics, is first attested in 1961."
In 1964 when King Curtis & The Kingpins recorded the song "Soul Seranade" that title referred to "Black music" (or "African American" music if you want to get specific). In 1967 that same group recorded "Memphis Soul Stew". R&B singer's Arthur Conley now classic 1967 record "Sweet Soul Music" also referred to African American music.
In 1968 when the Fifth Dimensions recorded "Stoned* Soul Picnic", they were talking about having a picnic-Black folks style. This was back in those days the word "stone" was used by some hip African Americans as an intensifier. "A stone soul picnic" meant "a picnic with a lot of soul food & soul music, and probably also a lot of Black people-if not only Black people [in attendance].
UPDATE January 31, 2017
When I wrote this post in 2011, I incorrectly gave the title of this Fifth Dimension song as "Stone Cold Picnic".
It's possible that the word "stoned" in this song has the meaning "being under the influence of a drug (as marijuana)" https://www.merriam-webster.com/dictionary/stoned, i.e. being "high". However, I think that "stoned" might have been meant the intensifier "stone" ("absolutely"; "completely soul picnic and the word "cold" might have had the vernacular meaning "very cool" (very hip). Therefore, a "stoned soul picnic" might have meant a really good soul picnic.
I wonder if that song's composer Laura Nyro might have coined the term "stoned soul picnic" as a play on the old term "stone cold". (According to https://www.merriam-webster.com/dictionary/stone%E2%80%93cold
"Definition of stone–cold: absolutely
But perhaps Laura Nyro might written the title/lyrics "stoned soul picnic" to [also?] suggest that lots of people at that soul picnic might be having a really good time because they were getting "stoned" ("high").
-End of UPDATE-
The Fifth Dimensions also recorded a song entitled "California Soul". In 1968 the title for the instrumental hit single by Young-Holt Unlimited, "Soulful Strut", referred to the distinctive smooth, gliding walk that African Americans do when we want to walk that way. And in 1967 when the R&B singers Sam & Dave recorded their hit song "Soul Man", they were praising Black males. (But I don't think that the referent "soul man" of "soul brother" was or is used that often among Black folk. Instead, we've used the terms "brotherman" which was shortened to "brotha" which was shortened to "bros" or "the bros". The only common use of "soul brother" that I can think of is "blue eyed soul brother" which means a White male.)
Those are just a few examples of African American music that includes the word "soul" in their titles. Moving to African American "lyric" from another genre, when the verse "soul sister #9/"sock it to me one more time" was first chanted in the playground rhymes, that verse was talkin about Black females. And "sock it to me" didn't mean punching either, but that's a whole nother subject. That said, it was my interest in that "soul sister #9" verse that led me to find out that the definition for soul sister had changed right in front of my eyes.
In researching the blog post that I published about the "Sources Of The Movie Big's Rap Shimmy Shimmy Coco Pop" http://pancocojams.blogspot.com/2011/12/sources-of-big-movie-rap-shimmy-shimmy.html, I happened upon several videos of non-Black girls performing handclap routines while chanting the rhyme "Soul sister #9" and I wondered if those girls actually knew what "soul sister" meant. I wrongly assumed that the original meaning for soul sister-a Black female-had remained constant for anyone who uses that referent. However, these entries from urban dictionary.com showed me how mistaken that assumption was.
"1. Soul Sister: Someone who fully understands you. Like a soulmate, but not someone you want to marry and make babies with. The sister of your soul."
-teenagedramaqueen Apr 9, 2006
2. Soul Sister
"The female equivalent of a blood brother. A best friend of the female persuasion that you consider family. Most times, the feeling is mutual."
-wolfpacleader1986 Oct 8, 2010
Using the "hip" spelling of "sister", here are two more definitions from http://www.urbandictionary.com/define.php?term=soul%20sista which surprised me:
"1. soul sista
"Your number one girlfriend, typically used if you are a girl."
-S1m0n3 Nov 9, 2007
2. soul sista
"A large black woman with an attitude and a deep singing voice. EX: Aretha Franklin, Whoopi Goldberg"
-Nick D Feb 9, 2004
With regard to example #2, I suppose it's beside the fact that African American vocalist (the Queen of Soul) Aretha Franklin was not always a large Black woman (one who weighs more than the American norm for women). And I suppose it's immaterial that Aretha Franklin isn't known to have a sassy attitude (which is what I think that definition means by "attitude"). Furthermore, I suppose its immaterial that African American actress Whoopi Goldberg isn't known for her singing. As far as I'm concerned, definition #2 is a big FAIL. However, definition #1 further points out how "soul" in "soul sister" has the same definition as "soul" in the phrase "soul mate" i.e. someone who really understands you; someone you are very close to.
The two sentences given for "soul sista" use what I consider to be so-called African American given names, fake African American English slang, and exaggerated African American grammatical constructions. The blogger writing those sentences may have used that writing style and those names to confer some Black coolness on the phrase "soul sista", even though he weren't necessarily talking about Black females in those sentences. Or he might have used that particular pseudo-hip writing style to imply that the "soul sistas" he was writing about were indeed Black females. I chose not to post the entire sentence given for example #2 for "soul sista" since it contains pejorative language (the n word) and profanity. But here's the complete first sentence and part of sentence #2:
Sentence example #1
Liliana: Did you hear about Dana? Good gossip!
Claudia: WHAT! Dana's my numba one soul sista
Sentence example #2
Latrell: "I hear you been poppin' LaKeisha lately. Got yo' self a soul sista I see."
Urban dictionary.com's definitions of "soul sister" and "soul sista" were a surprise to me. After reading those definitions, I submitted what I think is the original (meaning the Black 1960s) definition of "soul sister" to that website. However, to date, that definition has yet to be posted there. Here's an considerably expanded version of what I wrote:
"Soul sister" is a 1960s African American referent for Black females. Referring to all Black people as brothers and sisters reflects the over-arching view that all Black folks are family, connected to each other because of the oppression we face or potentially face.
"Sister" and "Brother" have long been used as titles of respect not just for elders of certain Black fundamentalist Christain churches but for all adult members of those churches. What may be less known is that since at least the late 1960s, "sister" and "brother" have also been used among some afrocentric African Americans as titles of respect, regardless of the person's age. For instance, in the 1969, when I moved to Pittsburgh, Pennsylvania, I was routinely referred to as "Sister Azizi" by the brothers -and following their lead- the sisters in the afro-centric circle of friends with whom I interacted. The title "Sister" prefaced my name in conversation in recognition of and respect for the level of knowledge of Black culture I had - or supposedly had - since I had been a member of a cultural nationalist organization in Newark, New Jersey which was led by the well-known poet, playwright, and activist Amiri Baraka [LeRoi Jones].
It should be noted that the title "Sister" wasn't always used to preface the names of the other women in that circle of afro-centric people, a fact that made me quite uncomfortable. Furthermore, I didn't like the use of "sister" in front of my name because it reminded me of the naming practice used for Catholic nuns. Needless to say, since I was single and 21 years of age, I didn't want to be thought of as a nun. I therefore insisted that the title "sister" be dropped, and my first name be used by itself. Eventually it was. I'm sure that the other sisters in that circle of afro-centric folks were glad that that naming custom practice bit the dust.
Years passed, years passed and since at least the early 2000s another use of the term "sista" is the term "sista/friend". "Sista/friend" is used to denote a female friend that is as close to you as a female would be if you both had the same parents. In other words, "sista/friend" means the same thing as the new "White" definition of "soul sister".
I believe what really popularized and continues to popularize that "White" meaning of "soul sister" was/is the hugely successful 2009 record "Hey Soul Sister" by the White American rock band "Train". Click http://en.wikipedia.org/wiki/Hey,_Soul_Sister for information about that record. Here's a description of the official video of that record from that Wikipedia page:
"The video was filmed in front of Chango Coffee at the corner of Morton Ave and Echo Park Ave in Echo Park, Los Angeles, California. The video intercuts images of Train singing with a woman walking around her apartment and a man painting the words to the song on the landscape."
It's significant that the woman featured in this video wasn't a Black woman.
I know that it's the nature of the words & phrases that their meanings can change over time. I also know that different populations within the same period of time can have different meanings for the same words & phrases. But I thought that "soul sister" was somehow immune from those changes, just like "having soul" and "soul music" and "soul food" would always refer to Black folk's cultures. But now I realize that it's likely that those definitions might also change, or maybe they have already changed. And I admit that something about those changes depresses me.
Maybe it's selfishness on my part, but I don't want to share every single last feature of African American culture with everybody to the extent that there is no more African American culture. Maybe I'm wrong, but a definition of "soul sister" that means "a female soulmate" seems to be like whitewashing blackness, especially if people forget what the "original" meaning of "soul sister" was & is (with "original" here meaning from around 1961).
UPDATE December 24, 2011
In response to a summary of this post that I added to my facebook page, an African American male wrote that he considers the "close female friend, or female soulmate" definition of "soul sister" to be a trend and a co-opting of Black culture.
While I personally dislike the fact that the definition for "soul sister" had changed among some populations (I think mostly among White people & some other non-Black folks), I realize that it's natural for the meanings of words & phrases to change or expand. I also think that the definition of "soul sister" as a close female friend is more an expansion of the phrase "soul mate" than a co-opting of the term "soul sister". Furthermore, partly because of the huge influence of the record "Hey Soul Sister", I don't think the expanded meaning of "soul sister" is a temporary change as the word "trend" suggests. However, I think that African Americans will continue to confer our meaning to the referent "soul sister" while White Americans confer their meaning to that referent. This prediction might be wrong, but I hope it's not.
(These videos are presented in no particular order.)
Example #1: Sam & Dave - Soul Man
bluesouland, Published on Apr 18, 2013
Example #2: Arthur Conley - Sweet Soul Music.flv 
rick shide, Uploaded on Nov 11, 2007
Example #3: King Curtis & The Kingpins - "Memphis Soul Stew"
Uploaded by McGarVision on Feb 8, 2008
The only band that could make Booker T & The MG's sweat. "And now we need a pound of fat back drums..." King Curtis was inducted into the Rock and Roll Hall of Fame on March 6, 2000.
Example #4: The Fifth Dimensions - Stone Soul Picnic
johnrigs654321, Uploaded on Sep 3, 2010
In this title, the word "stone" is a (no longer used) African intensifier from American English, meaning "really" soul[ful] picnic.
Example #5: Train - Hey, Soul Sister
Uploaded by TrainVEVO on Nov 14, 2009
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