It draws 44,000 hits from Google, so it is by no means moribund, though nowadays not so common and it may be frowned upon. The meaning is clear. Answer says it has to do with a weak member of a group. Not sure how they came up with that angle. I see it as a comment on character. “Twisted sister” could be its (or her) progeny.
Wonder why specifically a sister. Sister city, sister stud (strange enough, along with a cripple stud), things are sistered on to, etc. Brothers don’t get the same treatment. I guess for the same reason a ship is a she. How old might the phrase be?