First, the clues you all missed... 
 This sister is not one of those sisters. Y'all should have gotten it the first time.   C'mon now--you saw her face, now let's have her name. 
 
Now the right answer (and a little behind-the-photo bonus)... 
 In the early-1970s, I sold a film option to Bessie to Porter Bibb, who was then the publisher of Rolling Stone and Associate Producer of Gimme Shelter, but has since made a name for himself in the financial world. Porter(and, presumably, still is) was a terrific guy, but I’m afraid he suffered the limits of most white Americans when it came to black people. No, I’m not talking about racism, or anything of the sort, but rather about their limited awareness of the black scene.  
That’s how Porter hired Melvin Van Peeples to write the script, based on my book--Melvin was still cashing in on his controversial film, Sweet Sweetback's Baadasssss Song, so his was a name white people knew. Porter also picked someone to play Bessie, a truly wonderful person who was very wrong for the part.  
In typical Peeples fashion, Melvin came up with a rather off-beat scenario wherein Bessie was the heavyweight champion of the world, between Jack Johnson and Joe Louis. An interesting idea, but it wasn’t going to work, and the screenplay draft was pretty dreadful. The result was that Melvin was taken off the project, and the screen writing chores given to Horton Foote.  
In an attempt to avert that change, Melvin took our star to a photo studio and had this picture of “The Champeen” taken. The lady is    Roberta Flack  
...who may never speak to me again