Weak Sister Meaning

(chiefly US, idiomatic) A person or thing which is the least robust or least dependable member of a group.

Example: 1964, "Political Thriller" (film review of Seven Days in May), Time, Feb. 21:
  "Your course of action this past year has bordered on criminal negligence," he barks to the man whose job he means to take. "You're a weak sister, Mr. President."
1991, Larry Reibstein, "A Mexican Miracle?," Newsweek, 20 May:
  San Antonio has always been viewed as the weak sister of the big regional centers of Houston and Dallas.