Brother from another mother!
You know, as famous as this saying is, I don't know that I've ever heard anyone say "Sister from another mister." Clearly it's been said as google verifies, but I think it's a pretty startling indictment on our society, which says either:
1. We just don't listen to women or don't let them talk, or:
2. Way too many women don't know who their fathers are.
I'm leaning toward 2, since "Who's your daddy?" is another very common phrase.
This has been another episode of "The Sociology of Slang".